People in this thread really need to take a step back and chill. I have a 4.5-5 inch mudminnow in a 2 gallon bucket full of moss with no filter and he's healthy and happy. It's not a permanent home, he's quarantining for columnaris and fungus he caught due to a long transport time. The columnaris showed up on him in a fully cycled 40 gallon breeder with a few spotfin shiners and large northern redbelly dace. I put him in that bucket with a bunch of moss, no filter and some salt so I could at the very least isolate him from the other fish and reduce the chance of the inspection spreading. He's been in there for 5 days now and the columnaris and fungus are completely gone. Some people would love to crucify me and throw me under a bus for temporarily having a 5 inch fish in less than 2 gallons of water, but if the conditions were terrible and he was unhappy, would he heal up as quickly as he did? He's eating like a pig and gets a 30% water change every 2 days.
The guy's trout is only 3 inches and it's in a planted 50 gallon - I'm sure it has enough oxygen. A healthy cycled tank with lots of plants can host a surprising amount of fauna without the system crashing if water changes are performed regularly.
Thank you! Anyway I finished the filter today and they are still alive.