Sorry for being slightly off topic here, but, unlike most other fish, most mbuna actually grow larger in captivity, by about 20-25%...research suggests that it might be due to the tendency for mbuna to be fed more protein in captivity than they get in the wild. However, most mbuna do actually eat some protein in the wild, some more than others, (like Labidochromis caeruleus), primarily tiny freshwater crustaceans.
In regards to clown loaches, and most fish for that matter, I think area, not gallons. My loaches are kept in tanks that are 5 feet long and heavily filtered with lots of water changes. They are happy and growing. Technically, any environment we do for any fish will always be smaller than it's natural environment, even for a neon in a 1,000 gallons! I try to be reasonable in the tanks I put my fish in and in the fish I choose. I house my fish in as large a tank as possible in relation to their likely growth in captivity, with the highest water quality and food as possible. Like so many things, balance and reason is a good guide.