[IUnknown]

I've been running a wet dry filter along with a canister filter because my canister filter was not turning over enough. The wet dry filter pump died on me when I did a water change today. I didn't think about it, but a couple of hours later and all my fish were dead. I tested ammonia and it did spike, but not above .5mg/l. I'm assuming the O2 is what killed them, even though they did just fine with the canister filter alone previously?

My ottos and amano shrimp did make it. I've raised the spray bar so its splashing into the tank. In general is there any way to improve oxygen with a canister filter?

 

[IUnknown]

So looking back, I think having two canisters was deadly. The bacteria colony was large because of being well oxygenated by the wet dry filter. When just the canister filter started back up, that large colony sucked all the remaining oxygen out of the water column and killed all the fish. I would have been better to turn the canister filter off until the replacement pump for the wet dry filter arrived.

 

[Pattern8]

This is interesting. If there was still surface agitation occurring some rate of gas exchange must have been happening. Are not wet dry filters recommended against in a planted tank? Is it heavily planted and if so is it possible that the plants were somehow actually responsible for oxygen consumption? If you had a slight spike in ammonia would that not suggest a lower colony size?
Also, and maybe someone with actual experience with this problem will enter in, but from what I've read fish who die from affixiation, whether it be from ammonia or critical oxygen levels, will present a classic death pose of gaping mouth? Do you remember if this was so in your case?