I have a 55G that is about 10 years old that used to be part of a centralized system that I used to grow-out baby discus and angels in - it has a hole drilled in the bottom corner. I now use this tank for my planted discus tank, and put in a 1" PVC into the bulkhead at the bottom of the tank that reaches to the top of my water level. I did this because I love the way it allows me to "skim" the top and keep the surface clear. When its not skimming the surface gets gross and stagnant.
But the sump has always been horrendous
at degassing CO2. Providing and maintaining any good level of CO2 has always been a challenge. Water flows into a Durso (overflow silencer) down the 1"PVC into my sump - the faster the tank water turn-over, the more bubbling/splashing in the sump - and the more CO2 I needed to add to the tank.
I did all I could with multiple CO2 reactor designs, both in and out of the tank (hey, I built a couple of really nice ones...) but still could not keep a decent consistent
level of CO2 in the water. I was putting in 4-5 bps with full dissolution in the reactor, and still having a pH of 6.9-7.2!
I eventually bought a Miliwalkee pH controller and that helped enormously in getting and keeping the pH down - but the bubble rate required to keep adding CO2 to the reactor/tank water was so fast (to keep up with the degassing in the sump) that I would go through a 10lb bottle of CO2 every couple of months - and still had varying CO2 levels (indicated by the pH meter).
Tom recommended that I "Seal" the sump - but because it is a glass tank and the way my set-up is - sealing the sump itself would be a challenge and make fert dosing a pain (I just throw the chems in the sump - having to unseal/reseal with my setup would be very complicated (but not impossible))
So I finally figured out how to seal the sump in an in-expensive and cheap manner - and its proving to work very well (I may be premature, but since I made this modification four days ago my CO2 usage has been much
less and the pH has been very steady at 6.1-6.2 (before it would vary from 6.2 to 6.9 depending on the flow through the sump).
The answer was cheap and simple - The "Drain" coming from the tank now flows into a 1 1/2"x12" clear PVC that (literally) looks just like a gravel cleaner. The 1/2" tubing end of the "gravel cleaner" comes from the main tank is sealed tight to the tank drain, and the wide PVC end of the gravel cleaner is 2-3" under water in the sump. https://www.aquaticplantcentral.com/g...php?i=2941&c=3
In this configuration my CO2 controller actually shuts off now and I have a much lower bubble rate - pH is constant 6.1-6.2 (kH is maintained at 4-5) and the meter only turns the CO2 on a couple of times an hour (for 5-10 minutes or so). I am hoping a long time frustration has finally been solved....
Hope this helps anyone with a sump who was having similar issues...