Built in overflow modification


Lifetime Charter Member
Lifetime Member
Jul 27, 2007
New Hope, Pennsylvania
This is my modification to an overflow chamber I've been using for about 9 months now. I've always disliked in the past the varying levels in the overflow due to evaporation and the amount of degassing. IMO its more of a hinderance in a planted tank and a good steady surface ripple yields more stable results. And this may help others and possibly spur additional ideas on how to utilize a built in overflow to suit ones needs. Or I just like to tinker far to much :cool:

So after some pondering and a few lightbulbs later.

I use plastic to cover all but one of the teeth, this gives more than adequate surface skimming, and as already noted I rely on a good surface ripple across the tank for gas exchange.

There is a 1.5" bulkhead port that is open. This allows any fish/shrimp access out and also gives a good amount of surface movement.

Below that is a coarse sponge filter to keep any critters and leaves from getting into the pump.

Below that is a 1.5" bulkhead port that has a flat screw on screen. This is rated at 90 gpm. This is also drilled at my 50% water change height to make water changes a little easier.

I've been experimenting with a diffuser in this lower chamber as a quasi reactor. For the last month I use 50/50 Needlewheel and this reactor with about the same results as 100% Needlewheel. But I use quite a bit of gas exchange in the form of surface agitation and extra CO2 to compensate, so getting concrete results is hard to nail down. So far though 99% of the CO2 dissolves in this chamber, since most of the bubbles get caught in the sponge and dissolve before they can escape. Late in the afternoon I will get some burping of any built up gas.


Tom Barr

Staff member
Jan 23, 2005
I use a disc in one aquarium, then a CO2 fed into the sump return pump.
This means there is good CO2 in the tank, and also in the return water.
A needlewheel can remove the disc.

My other tank has a sponge prefilter then a screen, then goes to in line canister filters/heaters etc, and no overflow built in prefilter etc. I've found better results with Hang on prefilters+ sumps and planted tanks, use less flow and just get the surface cleaner, then most of the flow use a powerhead inside the aquarium, then perhaps another canister filter.

Built in overflows always seem louder, more fish etc get trapped, takes up more space etc. If done externally, then the built in overflows seem good, same with bulkhead loops for the canister filters.

So a mix of both in line canister and a small surface skimming wet/dry seem ideal.

Tom Barr


Lifetime Charter Member
Lifetime Member
Jan 15, 2009
Slight modification you might try. I haven't done this yet but I'm strongly considering it with my biocubes as I'm tired of fishing cherry shrimp out of the overflow...

take a solid plate of plastic , cut down 1/4 round of a pipe or similar that is the width of your tank. Place the bottom edge below the teeth of your overflow and right up against the back wall to form a seal. Take the upper edge and move it out a bit until you are just below your desired water level. Instant coast to coast overflow. Just make sure the plate is level from side to side. Whether that means the tank being level or the plate being level to the water is up to you.

Most of what I've been reading on this lately suggests that the longer distance allows for a "thinner" amount of water to pour over the edge resulting in better skimming. For example, if your current overflow were 3 inches wide and you could measure 1/4 inch of water thickness, then for the same flow at a 12 inch wide tank/overflow you would now have 1/16" of water. The thinner water layer also makes it less likely for fish to jump over or get pulled in. Better skimming also means you won't need much if any surface agitation as the whole surface is dust/scum free and constantly moving. All the water movement could be done under the surface if you wanted. You could also replace any teeth in the original overflow with some mesh or grid material or even some coarse DLS/prefilter sponge.

Longer overflows also seem to incur less noise and any type of overflow can be done with less drop. The "Herbie" standpipe designs make this even better as you can get by with very little vertical drop if desired and little or no air induction into the standpipe to cause issues. The internal overflows like this seem to be commonly known as calflo designs I think. They're more of an internal box design but that seems to be more to allow bulkhead fittings to be used inside. Since you already have the way to move water out of the overflow you should be able to use a flat plate instead.