Retrofit overflow skimmer

brianhama · Jan 24, 2024

I recently purchased a used 125 gallon acrylic tank and I’m using it as my first salt water tank. Being new to salt water tanks, I was oblivious to the entire idea of a sump and an overflow skimmer. The tank I bought is designed for dual canister filters. There are two pairs of intake and return located on the bottom of the tank, center (front to back), on both sides, (left to right). They are right in the middle of the tank and look horrible. I’ve tried to cover them with rock, but I suspect that’s going to mess with the water flow.

I now realize that I’d much prefer to have a sump and overflow. Unfortunately, I have not only begun cycling and stocking the tank, but I had it built into a wall in my home, so it’s not going to be a simple thing to replace at this point.

Is it reasonable to retrofit an overflow skimmer and sump on a tank that has already been plumbed using bottom drains like this? I can relocate the few fish I have and empty the tank if need be. Honestly, if I can’t retrofit the tank, I’ll probably replace it, but that’s going to be a nightmare and I’d prefer to avoid it.

I should mention that because of how it was built into the wall, I don’t think a HOB overflow is doable. The tank has a built in acrylic lid surrounding the top of the tank. Then there is about 6 inches of wood directly above the tank surrounding the entire tank.

Thanks for any suggestions.

MarineandReef Jaron · Jan 24, 2024

The biggest difficulty you will find is that the easiest way to add an overflow it to use some acrylic glue to add an overflow weir to the back of the tank like this one. https://www.marineandreef.com/ProductDetails.asp?ProductCode=RCPR20418

What many people want to do is make a traditional overflow weir that goes from the bottom to the top of the tank. This is very hard. You will probably need to have a piece of acrylic custom-cut to the exact right height. It can be done but it is very difficult. If it were me. I know of some local acrylic manufacturers in my area who could modify the tank with an overflow but they would want me to drop the tank off at their shop. It might be worth it to spend $600ish drop off the tank and have it professionally buffed and an overflow installed. It would come back looking like a brand-new reef-ready tank. I personally wouldn't try installing a top-to-bottom overflow myself.

Gtinnel · Jan 24, 2024

Having holes in the bottom of a tank for the overflow is not uncommon. The holes will have a standpipe coming up from them and then a overflow box around it. You will often see this style on cheaper rimmed aquariums and the overflow boxes will be in the corner(s) or centered on the back wall. If I understand correctly your holes are on either side of the aquarium and centered between the front and back, which in this size tank would probably rule out having the overflow there. I have seen tanks that have the overflow in the center, but they are generally massive tanks.
If it were my tank I would probably drain it and find a way to plug those holes on the bottom, either with a piece of acrylic covering them or with plugged bulkheads. Then drill a few holes on the back of the tank to use a traditional overflow box. Even if you had room for a hob overflow I would recommend against it, they’re not commonly used anymore because if they lose siphon it can cause your tank to overflow.

Also, some pics of the tank could be beneficial in understanding what can be done to add an overflow.