Help with Oxygen Depletion?

[Bubbletrouble]

I set up a 65 gallon tank about two weeks ago. I had intended to run an under-gravel filter into my internal filter and then back into the aquarium. The internal filter has a venturi valve on it. It worked fine for a day until I noticed my fish were breathing heavily all the time. It got so bad my Maison Reef chichlids were lying on their sides at the surface, gasping for air. I disconnected the under-gravel filter and ran only the internal filter with the venturi valve. The fish were fine within a few hours. Then I hooked up an "outboard" filter with the intake connected to the under-gravel pipe. Same issue. Fish were gasping for air within a few hours. Disconnected it from the under-gravel and ran it by itself, and within a few hours all was fine. Last try was running the outboard filter by itself, with a powerhead connected to the undergravel filter tube. After about six hours the fish started to breathe heavily again, congregating at the surface. I disconnected the under-gravel again and just running the outboard and the powerhead as a circulator. What's wrong here?? Surely the bacterial build-up in the gravel can't be that bad yet after two weeks?? Any ideas? The fish are so far okay. No losses.

[gabdewulf]

Add an airstone/fixture?

[Bubbletrouble]

I'm not so much concerned about how to fix the problem; I have put the under-gravel filter on a timer for now, running it for a few hours a day. I am just wondering if anyone else has encountered this issue?? This is not my first aquarium, and not the first under-gravel either. This has never happened before.

BTW - ran the powerhead and outside filter overnight without the under-gravel, and all the fish are fine.

Edited September 11 by Bubbletrouble