Um...not sure where to start....did you first cycle your tank? or did you buy the tank and then put fish in it directly? If that is what you did you are cycling your tank with fish. You might want to read up on the nitrogen cycle on this forum. There are MANY discussions concerning it. Just type in nitrogen cycle in search bar. Now... from what I can tell, it sounds like your tank is overstocked. A 5g hex is very small for that many fish. Do you know what kind of pleco you have? Even if it is one of the dwarf variety, a 5g is way too small and not enough footprint for him to be happy. Same with the catfish I am afraid. You could keep mollies or guppies in this size tank, but not many. You need to get an API (Aquarium Pharmaceuticals) liquid test kit (not paper test strips) with glass vials for testing your water. That is every fishkeepers best friend. You need to know exactly if it was nitrites tested or nitrates. My guess is that it is nitrites and your tank is not done cycling. You need to do a huge (50-75%) water change and get that down. When you say you "cleaned off your filter" what does that mean? You might have killed off more beneficial bacteria that the tank was trying to establish. If you rinsed it off in tap water you definitely killed off beneficial bacteria, as chlorine kills it. You always rinse your filter in used tank water or dechlorinated water. Is there anyway you can take your fish back to the petstore so you can complete your cycle without fish? If not, you will have to do daily water changes (25-50%) to keep water tolerable for the fish. Also, as I said before, I think the tank is overstocked anyway and you need to return some for that reason or get a bigger tank, cycle it, and then put the fish in the bigger one. Hope that helps. Keep asking questions. This is a great forum! Welcome to AA!!