I'll try not to sound too newbie-ish in my reply but I think your fish are in trouble. It can take weeks, if not months for a bio-filter to establish its self. Nitrite is extremely toxic to fish. Anything over 3 ppm is in the death zone. The only way to reudce the level of nitrite is to reduce sources of ammonia. No ammount of water changes will do it on its own!
Things to do immediately...
1) Increase your pH to 7.0 - 8. Nitrifying bacteria prefer an alkaline environment. This is in contradiction to your tropical fish who prefer a slightly acidic environment.. however they will acclimate to the higher pH if done slowly enough (no more than +/-0.2 per day).
2) Increase your water temp to 80 - 84(F). Nitifying bacteria metabolize more quickly with higher tempuratures. You can do this right away with no harm to your fish.
3) Remove all but 1/4 of substrate from your tank. Typically substrate is used as a place to grow beneficial bacteria.. however with a bio-filter (esspeically two) this is unecessary. All substrate does in a tropical tank with a bio filter is provide a place of things to decay and release ammonia. Keep a quarter inch for it to look nice, but any deeper and your catfish cannot do his job effectively. Stir the gravel every other day to allow sediment to be sucked up by the filters. Replace the particle filter pads if you havn't done so recently.
4) Add an airstone to increase the oxygen content in the water. Nitrifying bacteria are Aerobic. They require oxygen. The more they have to fight for their share the slower they will convert the highly toxic nitrite (NO2-) into far less toxic nitrate (NO3+).
I think the situation is salvagable. Quick action will ensure the fish won't suffer too much. Just be careful when adding chemicals such as pH uppers so as not to shock the fish. Maybe it's just the newbie in me, but I dont' see anythign wrong with keeping an overstocked tank so long as you're careful about it. In hindsight you should have added the section bio-filter first, then the fish.
If you're up for spending some more money though, I highly recommend Bio-wheel filters (Penguin or Emporer). These do so much better becuase the wheel is in contact with oxygen in the amtosphere. Bio-filters that are constantly submerged are not as effective.
That said, I'm not a expert.. just an enthusiast such as yourself. I think everthing I've said is accurate and I hope it helps. But I welcome correction at any time.
