I have a 29 that is filled with nano fish and to be honest it could use an ac 70 in addition to the 2 sponge filters it already has. I also have two 75 gallons that to be honest would do best with a sump given the high stocking.
I also agree with the others. I could put a dozen neons in a 10,000 gallon and oxygen concerns aside I’m not even sure if it would require a filter while it I put an Oscar in a 40 breeder (I am using this as an example, I am not opening up conversation about it) then it would require heavy filtration along the lines of a canister or better yet a sump.