I don't live in a place where I can just go back to the store. Is there any way I can keep them in that tank together? Like more frequent water changes, etc. One of the neon tetras has since died, so that brings them down to two. Can any of them be taken out and put in an unheated, unfiltered tank? The guy at the pet store was obviously stupid, but he said the pleco would only grow to be an inch or so long.... is there a type of them that this is true for?
No way! For one, both goldfish are coldwater species and cannot be kept with tropical species. Also, more water changes will not help your situation that much, as goldfish are poop machines and produce enough waste to require 5 to 10 times the filtration of a regular fish. Your fish will die from poor water conditions and if they don’t, they will die of organ failure. When fish are kept in such a small tank, their growth is stunted and their organs are either compressed or they explode through the small skeleton. Those fish cannot be kept in unheated or unfiltered tanks. Goldfish do not need a heater, but they NEED a filter. All plecos will get fairly large and create a ton of waste. Buy two large tanks from craigslist or find a way to take all the fish back. Never trust pet store employees! Only rare few are actually experienced.