IMO - too many people by a tank, regardless of size and then say OH, I want that fish not knowing if is the right tank for the fish. They do not want a bigger tank, so the fish grows and grows and is a really unhappy fish. Additionally, people tend to overstock their tanks, because they don't like seeing all that empty space - OH there's plenty of room. I am guilty of overstocking tanks, but then I overfilter those tanks and increase water changes. I feel that people do not research the fish they are buying BEFORE they buy it, bring it home and it grows and grows, Tiny oscars are so cute and adorable and personable, give me that 55 gallon tank and 6 2" oscars, What is sadder is that the people at the LFS don't stop them!!! CHA CHING SALE!!! Why would a fish store sell 1" red belly pacu for $1.79 each with out a big warning sign on them? The LFS should take some of the blame for unhappy fish. IMO if you are concerned that the tank you have might be too small for the fish, it probably is! If you do not have the funds to buy a suitable tank for the fish you want, DO NOT BUY THAT FISH! Buy the biggest tank that you have space for or can afford, and stock it properly. If I rescue one more 10"RTC from someone's 55 gallon tank, i am going to the fish store they bought it from and yell at them! Everyone is going to have their own opinion about minimum tank size, what a general rule is, I have mine too. The fish have to be happy and unstressed.... I have a great idea, I have a 10 gallon FOWLR with a percula clown,in it, I saw a really cute 2" Black Volitan Lionfish, I think I'll put him in! Can't wait to clean the sand!!!!!