I tend to use the information on SeriouslyFish to check whether a fish is suitable for a certain tank size - but of course it helps to know the name of the fish first which doesn't help people just looking for ideas!
SeriouslyFish mostly gives tank sizes in length rather than capacity and there are very few (if any) fish that they recommend for any tank under 45cm. Even bettas and endlers are listed as requiring 45cm. I think this rules out most tanks under about 30L as suitable for anything except perhaps shrimp.
I don't know how accurate SeriouslyFish really is or how many people would agree with that recommendation.
I used the list mostly for ideas, but of course enjoy doing my own research. Sometimes I find seriouslyfish's tank requirements a bit difficult to understand because I am awful at numbers and remembering the dimensions of my tanks. Plus the search function you can use doesn't work for me (profile search works well, but when I use the search where I can input my tank stats, it never works).
It would be nice to see a guide for the various types of fish suitable for a five to twenty gallon tank. I, personally, feel that anything under five gallons is really cramped unless it is an emergency or temporary situation (I've got one betta in a 2.5, but I'm trying to move him out as soon as I can), or if you're using a really nano tank as a food source. I keep snails in the small tank, get new ones every week, pull a few out daily before I put them in the ten where they will be eaten.
Maybe we could compile a thorough list of fish who can thrive in small aquaria. Single bettas in a five or higher, a few guppies in a five or higher, dwarf puffs, etc. Maybe if they are not listed by as bare minimum requirements but by thriving requirements, maybe then we could have a nice compilation for people like me who like small fish for small tanks and more small fish for bigger tanks.