Fluval Edge Pre-Filter Mod for Spec Aquariums

One tip that I offer on my page on keeping Betta fish in a Spec Aquarium is to put filter foam over the return nozzle to help reduce the fast currents in the Spec aquarium. There are probably several situations where the fish/inverts might benefit from reducing the very fast velocity of water that comes from the stock Fluval Spec Nozzle.

For those that don’t want to make something from scratch, I had suggested you use a Fluval Edge Pre-Filter Sponge on the nozzle.  I decided that I should try this mod out to see how it works.

Fluval Edge Pre-Filter Sponge in Box

Fluval Edge Pre-Filter Description:

The part is very simple.  I’m not sure what exactly it does on the Fluval Edge aquarium, but it is a simple cylinder shaped piece of coarse, black foam.  It has another cylindrical cut-out in the center, but not all the way through.

Fluval Edge Pre-Filter Sponge

It is a bit larger than expected.  I measure my sample as:

  • Diameter: 40mm (1.57″)
  • Length: 70mm (2.76″)
  • Hole Diameter: 15mm (0.59″)

How to Install:

Gosh, the simplest mod yet.  You just put the hole in the sponge over the end of the nozzle.  Done.

Fluval Edge Pre-Filter Sponge mounted on nozzle of Spec V

Just like that, flow goes from fast and harsh to slow and gentle. The only real trick is that it tends to float a bit when it is dry.  First dunk it in the aquarium and give it a squeeze under water to get the air out.  Then it will be less buoyant and will rest in a neutral position.

Performance – How Well Does it Work:

As a simple modification to make your flow hospitable to a betta or other sensitive fish, it works great.  The flow velocity is greatly reduced. The picture below shows the stock nozzle.  You can see the ripples in the water that hint at the flow speed from the nozzle.

water ripples on surface of Spec V Aquarium from Stock Nozzle

The picture below is with the pre-filter sponge over the nozzle outlet.  No more ripples.

Water pattern in Spec V Aquarium with Fluval Edge Pre-Filter Sponge on nozzle

Even better is this video. It shows several angles of the tank with and without the foam diffuser:

The velocity is absolutely tamed with the piece of foam installed. I questioned if the flow rate (actual quantity of water) was reduced, but I don’t think it is; possibly just a tiny bit.  It is really just a change in quality of water flow, not quantity.  The stock nozzle is like holding your thumb over then end of a garden hose.  With a sponge over it, it’s like a slow spray wand attachment on a hose. Same flow, just diffused in all directions instead of blasting in one focused jet.

Fluval Edge Pre-Filter Sponge inside Fluval Spec V planted tank

For those that want a more gentle water pattern in their Spec for little effort, this is a good option.  The only negative is that the Fluval Edge Pre-Filter is actually quite big for the scale of this tank.  Ascetically, a smaller sponge would be better, but it is still not all that bad.

49 thoughts on “Fluval Edge Pre-Filter Mod for Spec Aquariums

  1. I can’t tell you enough how much this website has helped me with my Fluval Spec V. Probably the most helpful was the flow tube and pre-filter mod. I was a given an African Dwarf Frog in a tiny acrylic box as a gift, and decided immediately to upgrade him to the Spec V. The tank as it comes is not well-suited to ADFs, but your mods made it perfect! The holes in the flow tube solved the temperature issue I was having with the heater being housed in that compartment. As it was, the flow from the return nozzle was too violent for the little frog even when on the lowest setting. I put the Fluval pre-filter sponge on the return nozzle and now the flow is so gentle. He doesn’t have to fight the current to reach the surface, and sometimes he even hides between the sponge and the glass! I haven’t found any issue with filter performance. Perhaps the only issue I found is that the filter is a little noisier with the holes in the flow tube, but not to the point of being too loud. Thank you again for your help!

    1. Thanks for the comment, and glad the information is helpful to you and that your frog has a happy home!

  2. I’m enamored with this blog and have used quite a bit of your advice as I have started my own build. I am having issues with this piece though. I have the sponge on the nozzle, but it seems to cut the flow down to zero. I am wondering if the water is stagnant as I don’t see any movement going on at all. I’ve taken it off a couple times and since I haven’t put any holes in the return tube, the flow is pretty decent (even though I turned it down all the way). With the sponge blocking the outlet nozzle, do you think there is still enough water flow?

    1. I understand what you are seeing. It does seem that the flow goes to nothing. However, I bet that the flowrate is about the same with and without the sponge over the nozzle. It’s just that the flow is . . . different. Kind of like an adjustable showerhead that can go from a piercing jet to a gentle shower. The flowrate is about the same but the dispersion is different.

      If you drop some fish food near the overflow slots, does the food go into the filter section? That is a visual that there is still good flow in your tank (not zero).

      I take it you are doing this for a beta or other fish that doesn’t like high flow?

      1. Actually, I’m setting up a highly planted tank that will eventually be for a dwarf puffer or a pair of clown killifish (although after looking at some high-end beta fish in San Francisco a couple weekends ago, I haven’t completely ruled that out yet). I’m still in the process of planting assorted plants and having them grow and fill-in. I baffled the flow so that the plants would have a chance to put down roots and not get ripped from the substrate by the current (the dwarf hair grass was having a heck of a time). I also have some duck weed on top and I like the long root systems that grow with the current baffled. So, all that being said, I haven’t started feeding anything yet, however, the floating duck weed doesn’t move around much in the tank and I’m starting to wonder if I should just drill holes in the return tube (your other mod suggestion) and let there be a slight stream. I think once the plants get filled in, the stream won’t be such a big deal, but for right now it seems very harsh without the sponge and non-existent with the sponge.

      2. Yes, sounds like you are in a tough in-between place. All I can add is to play with where that nozzle is pointed and maybe find a sweet spot where the flow isn’t pointed at plants that are easily uprooted.

        Then again, I had a heck of a time keeping things down at the start so it’s probably a universal challenge no matter the setup.

        Good Luck!

      3. My tank is having the same problem!! I am wondering if you use the sponge if you need to put the flow on something a little higher than the lowest setting? I keep having to go in everyday to re-adjust the sponge to get the flow back. Although too much flow power pushes the sponge off the nozzle. Any recommended solutions to this? I have a long-finned half-moon betta so I know swimming wise he can’t have too much of a flow. TIA!! 🙂

    2. I move my filter speed to the highest setting when I have the pre-filter sponge on.

  3. Hi Nate, just wondering if you considered cutting the length of the foam down so the cutout length just the same as the nozzle length. This could make the foam look smaller and more aesthetically pleasing.

    1. That did not occur to me, but I think some length could be cut off to make it smaller and it would work about the same. Give it a try and let us know how it turns out – Thanks!

  4. Hi. I have the Fluval Spec V with a betta and live plants. As per your advice, I placed a prefilter sponge around the output noozle. It does a great job of greatly reducing water flow in the tank and my betta is happy. However, I’ve noticed that I often see a white film (protein scum?) develop on the surface of the water. I always try and remove as much as I can, but it always comes back. Do you have any recommendations on how I can solve this problem? Will Purigen or an air bubbler help? Thank you!

    1. I don’t think purigen will help with surface scum. The scum buildup is a common issue (among many aquariums) and I can’t see a great solution with the stock spec setup, especially with the gentle flow with the nozzle covered up with a sponge. One thing to try is to lower the water level a bit so that flows faster through the overflow slots.

  5. First of all, awesome site buddy. I got plenty of useful insight through your website. Also, another great place for people to talk about their fluval’s and experiences that come along with them.

    I added this mod and it works like a charm. I cut off as much as I could and I set my filter to the highest flow setting. I thought my betta was the happiest he could be before this mod change, but I was wrong. He’s way more happier now and swimming around like a poaching ninja that he is. In regards to filtration, I do believe that it may set the fluval filtration back just a bit, but nothing to worry about if you’re doing water changes weekly like you should. Cheers, everybody.

  6. Awesome information Nate! Your site has been so helpful to me, thank you.

    I just got the Fluval Spec V for my Betta (after starting him off in a gallon bowl – eek – then moving up to a 3.5 gal bowfront, and finally settling on the Spec). I had some trial and error trying different configurations, and what worked for me is a little different combination than I have seen posted around the net, so I’d like to share in case it works for someone else, too.

    After trying a few suggestions, I settled on this configuration:
    – Set the pump to the highest setting to pull in water and debris better.
    – Cut three small holes in the pump’s output tube.
    – Made sure to position the pump’s intake area towards the filter media (the output tube does not naturally align with the hole for it up top, but it’s flexible and positions just fine).
    – Used the Fluval Edge pre-filter sponge on the output valve. I cut a hole in the end of the pre-filter sponge to increase flow (it’s still really low) and shortened it.

    With the pump set on low and no pre-filter sponge, the flow as still to high for my betta.

    With the pump set on low and the pre-filter sponge added, the flow was non-existent and there was no movement in the water at all. Nothing was being pulled through the top slots and into the filtration media. Even bubbles would pass right on by. I found it hard to believe the water would be properly filtered this way. It seems to me the pump needs to be set on high to pull water through the system better, and then you just have to adjust the output flow accordingly. So far, that seems to be working better – I can see water and debris getting sucked in when they get close to the slots now. And the pre-filter sponge works nicely to slow it down on the other end.

    So far, happy betta. Hopefully, happy human to follow!

    1. Excellent! I like the way you played with the setup and tried different things until it worked. Thanks for sharing.

    2. I did the same thing you did except I removed the duckbill nozzle altogether. If you look at the interior diameter of the duckbill you will see that it further restricts the water which would create a even stronger stream and also probably restricts the flow with such a small pump. So this is what I did to keep the newly shortened sponge in place without the duckbill… I took a very small zip tie and tighten it as much as I could around the portion of the elbow that sticks out past the wall a bit (where the duckbill attached). I cut the excess zip tie leaving about a 1/4″ tail (this is important). I then slipped the zip tie off the end of the elbow and placed the elbow through the hole as normal. I put the zip tie inside the shortened sponge hole. Now this is where it gets tricky… I pushed the sponge against the elbow and pushed the zip tie back around the elbow end (you have to feel for it with your fingers). The zip tie “tail” creates enough tension on the inside of the sponge to keep it in place. So now I have optimal flow and my betta, Simon, is happy as a…. well happy as a betta can be I guess.

      1. I had to read it a few times to understand, but sounds like a great mod and a great concept – get rid of the nozzle. Thanks for sharing!

    3. Thank you so much for mod info. I just found your info today. The first full day new betta has been in tank. He’s obsessed with filter. I just ordered the 340 20 mesh. I think this looks the best from pics. He’s a dumbo baby crown tail and I want to resolve ASAP.
      I have nozzle pointed upward, but do think I need to slow it.

      I don’t want to put holes in tube in case I opt to use tank for other fish in future.

      Can I just turn up pump and use Edge sponge?; If so, how should the duck tip be turned? Horizotinal or vertical? Pointing, up, down or straight?

  7. I see a small flow even without having to cut the other end of the sponge? Is it mandatory? if so how large a hole?

    Would adding the sponge still let the filtration system work properly?

    1. I’m not following, but remember . . . nothing is ‘mandatory’. Just play with things until it works well for you and your critters. If you modify the sponge and don’t like it, you can always not use it or buy another and try again – they are cheap.

  8. I have found a new solution to modify the output on the Spec tanks by using a elbow off of an old sponge filter I had laying around. It fits perfectly over the output in the tank side without using the nozzle. The opening is larger on one end, so it doesn’t shoot water onto your ceiling. This will create a great surface agitation without the current destroying your plants or pinning your Betta and/or shrimp to the walls. I turned my pump all the way up as well. I tried using the sponges but it does impede the flow somewhat at the pump housing because my tank temps got even hotter after I did this mod, suggesting that there was some sort of restriction even after drilling holes in my output tube. My Fauna will be happy with the new oxygen exchange I am sure. I posted pics on the write up at the plantedtank.
    I hope this helps someone out there

    1. You said you posted pics of your setup. Could you please provide a link. Thanks.

  9. I have a question for those of you that have done this sponge filter mod. I’ve got a Fluval Spec 3. My poor beta was getting pushed all over the tank with the stock filter set up on the lowest pump setting.

    I put one of these sponge filters on and it has definitely solved the problem. However, now I’m questioning if this is restricting the water flow too much. I’ve noticed there is no movement on the top of the water. I have lots of bubbles accumulating and they are not moving at all. I’m not sure if water is even moving over the filter or if water is simply being sucked in from the bottom hole in the divider (I should have filled this in with silicone).

    My question is, for those of you that have done this sponge, what setting is your pump on? Did you move it to a higher output setting or do you keep it on the lowest one?

    Does anyone find that the water on the top of the tank is stagnant and not moving around?

    1. Definitely upped the output on the pump to the highest setting. Also put a few holes in the pump tube to allow heater water flow. For the sponge, I poked a few holes in the material with a small bamboo skewer to ‘loosen’ it up so that there is slightly more flow. Still get extremely calm water on the surface but look at your filter – I have purigen in place of the charcoal and there is definitely water flow through the filter media. One tank I have the lower intake plugged with a small rubber shim that used to be sold at Walmart near the pain section- just cut it back and it fitz perfectly in the intake without any silicone. The other tank has the lower intake unplugged. Don’t really see a difference in flow over the top of the filter but would think plugging the lower intake make sense.

  10. What a Great Website! Kudos to the author of this website. Nevertheless, having raised bettas for over 50 years; the following are my experiences with bettas and the FLUVAL SPEC V. (1) Having observed bettas in Vietnam and Thailand–they truly are subjected to significantly more violent water flow than the SPEC V pump emits even at the high setting.
    The deltas, rivers, streams and rice paddies of this part of the world are subjected to severe storms which cause at many times very severe water turbulence. Thus, within the DNA of the betta is the ability to adapt to violent water turbulence.

    Specifically, have a seven year old crown tail betta that I have housed in a SPEC V for about a year now. This betta enjoys being thrown around at the medium setting of the pump. When I put the foam filter on the output nozzle, all he would do is hang around and on top of the output foam filter ALL day. Simply stated, this betta did not like the foam filter in place. After I removed the Foam Filter, it was evident that he enjoys the flow coming out of the nozzle. In closing, like us humans-bettas have their own unique likes and dislikes. Finally, the assertion that all bettas do not like and can not tolerate the flow rate emitting from the FLUVAL V output nozzle is respectfully false.

    1. I read your post and have been watching my betta today on and off. (Tank is right next to me on sofa and I’m on workman’s comp).

      I see a lot being done on the flow with many mods. I know Bettas don’t like a lot of surface motion, but I turned my nozzle a tilted horizontal, a little up and he doesn’t seem to mind.

      I have enough decorations, caves and plants if he needs a break he can find one. As you said you’ve raised Bettas for so long and I right for option out of filter out put mod or if the flow going to cause too much stress?

      Although currently he’s obsessed with filter intake. (I’m going to do 340 20 metal mesh mod as he’s a baby dumbo crowntail and I’m not about to loose these fins).

      1. Sounds like you are on the right track. You can certainly leave things as they are, play with the return nozzle angle, and see how your fish responds over time.

    2. John – I couldn’t agree more with regard to this not being a mandatory mod. Lots of people have given feedback that it helps with their Beta, but not every beta and tank will need this level of reduced flow.

  11. I thinks someone else here might have mentioned this but putting a plant in front of the nozzle is a great way to calm the flow in the tank and still get good circulation. I just realized this after dealing with some algae and stirring up the water. I wanted to increase the flow to get the particulates out of the tank so I removed the edge pre-filter and noticed that my betta was doing just fine. The water sprite is buffering the flow extremely well. Pump is set to high. After all the experimentation I think this is the easiest and most effective solution so far.

  12. Hi, when i first set up my spec tanks i noticed the water flow was a little strong even in the lowest setting. I keep dwarf shrimp and they like some flow but not that much. I like to have complete control over things in my aqauriums and I am very picky about asthetics such as equipment showing inside and out. So i decided I wanted to put a ball valve inline on the pump tube between the pump and output nozzle. It was actually quite hard to find the correct piece I needed, but I eventually found it and it works perfectly. If you are going to install it on a spec v then you need to remove the adjustment knob using some needle nose pliers or a small screw driver and pulling it off because the compartment where is the pump is is too small for the whole valve to fit. it fits in the spec III with no modification. So when you need to adjust it you just have to disconnect the output nozzle and pull the pump up a little bit and then stick the knob on the valve to adjust the flow. Also you will want to adjust the flow adjustment on the pump to the highest setting. That way you will have full control over the flow of the pump weather if you want it very slow or very fast.

    Here is the link for the ball valve:


    This valve fits perfectly over the tube without having to use zip ties.

    I hope this helps everyone as it seems like a better way to control the flow in these tanks.

    1. Thanks for commenting about this. I’ve intended to do this very mod – I even bought a valve. Never got around to it.

  13. Just wanted to stop by and say THANKS! I don’t have a fluval tank, but one really similar that has the same type of ‘back panel’ and nozzle set-up, although it is significantly larger than the spec V. Originally the filter output blew the sponge right off, but after stabbing it with a small skewer and cutting tiny holes out on the top and bottom it stays on and the tank is SO much better for my betta! A note on the question of whether this trick significantly reduces water flow- the temperature in my tank did not change, and it’s quite a large tank with the thermometer at the opposite end of the filter output. Even though a different tank model might make a bit of a difference, I think this would suggest the flow isn’t overly blocked?

    1. I think so. The filter mod changes the flow characteristic – not as straight line and more diffused. But, it doesn’t seem to drop the overall flow much.

  14. I cut an approximately 1”x1” piece of loofah material. Then folded it and rolled it and placed inside the tube between the elbow and the nozzle. With the door on the pump set halfway open, I get a nice flow and a happy betta!

    1. Cool! Lots of people use the ‘blockage’ method to good effect.

  15. I just started up my Spec V today. It will eventually be home to a Betta when it cycles. I can’t believe how the pre filter sponge for the Edge slows the flow down. Your web site has given me lots of great guidance. I’ve got a 29 gallon low tech planted tank but I’ve never had one this small.

    Any idea how long the pre filter lasts before you have to change it? I am using a blue/white poly pad on top of the sponge filter.
    Thanks for providing such a great resource

    1. The prefilter (over the return nozzle) should last almost forever. When you do a water change, put the filter in the old water, squeeze a few times to clean it out, and put it back on.

  16. I have a flex 34l. I put an eheim 300 compacton pump instead of the stock so I can control the flow rate. However, even on the lowest setting the flow seemed too stron as it pushed my betta to the other side of the tank. I have added the edge pre filter sponge over the output nozzle but then I have almost or really zero water surface movement. The food granules do not move at all. My question is- with the sponge on the output nozzle do I have to up the flow rate of the pump half way or to the max, so that I have decent movement/filtration without disturbing the betta.

    1. For sure – if you can adjust the flow of the pump upwards, give that a shot and see if you find a good balance.

  17. thank you for this easy mod! I moved my Betta to the Fluval Spec V (with the pump set at the lowest flow rate) and he could not get his food that was moving too fast. I unplugged the pump, but I think that this mod may be a better solve.

  18. Hi! This is a great site, thank you! The sponge I slipped over the nozzle keeps slipping off. Any suggestions for keeping it on?

  19. Hey, looking for advice with my 5gal spec. I have a betta in it. I have the pre-filter sponge on the tip and it’s not circulating the water at all. The top of the water has become very stagnant and there is a a lot of algea growing. I’m going to try to put a few holes in the tube and see if that works. But any other suggestions would be greatly appreciated

    1. * I forgot to add that, the reason for the sponge is the water flow is wayyyyyy high even at its lowest settings. He also gets sucked up against the side where the water flows in.

    2. Putting holes in the tube will make the water more stagnant (in conjunction with the sponge over the return nozzle – because the flow out the return nozzle will be lower). Not saying you don’t want to do this, but making you aware.

      It’s a tough balancing act – the sponge over the return nozzle diffuses the water flow to make it gentle, but by doing that, velocity is so low that you don’t break surface tension to break up the surface scum.

      Maybe play around with the angle of the nozzle/sponge (try pointing up at the surface) to get some surface turbulence.

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