I discuss a bit in my Fluval Spec V review about the design of the filtration system in the Spec series of aquariums and, specifically, what that little slot is in the back wall (toward the bottom). I’m going to try and explain why it is there, help others decide if they need it, and offer instruction on how to modify your Spec to close this bypass if you want.
The Spec Auxiliary Flow Slots – Purpose:
There are two auxiliary flow slots in the Fluval Spec line of aquariums. The first is about half way down the dividing plastic piece that separates the display section from the filter section. It is about 1″ long and the same width as the slots at the overflow up top. The purpose of this bypass seems straightforward: if the water level in the aquarium gets too low (from evaporation over time) the water would no longer overflow into the filter section and the pump would then run dry. By adding this little slot at the bottom, Fluval ensures that even if a user allows the water level to get below the bottom of the overflow slots up top, water will still flow into this little secondary slot and the filtration system will still operate.
The second bypass is near the top in the dividing plastic that separates filter section from pump section. It is horizontal and is right at the top of the stock filter media block. It’s a little less clear what this bypass does but I believe it’s still related to keeping the pump supplied with water. If the filter media was to become very clogged with debris, this slot would allow water to bypass the filter block entirely and flow into the pump section.
Reasons to Plug the Filter Bypass:
While the two bypass slots do add a level of safety to keep the pump supplied with water, I don’t feel that they add to the performance of the aquarium for how I will use it. If you are willing to keep the water level above the bottom of the overflow slots and are willing to keep the filter block reasonably clean, they serve no positive purpose.
The two bypass slots will allow water to flow around the filter media, which is not what I want. I want all the flow to be forced through the filter media to help clean debris from the water.
How to Plug the Bypass Slots:
The best way to close the bypass slots is to use aquarium safe silicone sealant. The reason you want to spend a bit more for ‘aquarium safe’ is because plain silicone sealant you get at the hardware store may have anti-fungal chemicals that are toxic to your fish and inverts. There are several brands to choose from; I bought aquarium safe silicone from Marineland and it worked very well.
The picture at the top of this page shows the lower slot closed off with silicone. It’s simple to do, just put a nice bead in the slot and smooth the display side with a wet finger if you wish to make it look more finished. Leave it to dry for around 24 hours.
I have also heard of people using a bit of foam or other material shoved in the slot to close it off. This could be a good solution for those who already have a working aquarium filled with water. The disadvantage is that the material would be more visible and may not completely close off water flow.
Waiting Till Later to Plug the Bypass Slots:
You may be tempted to get your aquarium up and running and then decide if you want to close off the bypass slots later. I’d encourage you to go ahead and fill them in with silicone sealant now rather than later. It’s extremely easy to do this with a brand new tank before it is filled. After it is running, you have to have the area very dry and it has to remain dry for a day after you apply the silicone. This will be very hard to do in a working aquarium with inhabitants.
If you decide later you don’t want the slots filled in, it would be very easy to remove the silicone with a sharp object and pick the pieces of silicone sealant out.
Alternatives to Using Silicone to Block the Fluval Bypass:
Since I wrote this, InTank Aquatics has come up with a product for purchase to plug the bypass in the Spec V and Spec Evo.
I have had several commenters below give creative ideas for plugging this opening. The most interesting was from ‘Louis’ who discovered these USB Dust Covers (who knew such a thing was even needed?) that fit in the opening. I haven’t tried it, but that sounds like a good workaround if the tank is already operating.