Seahorses need a tall coloum tank and are pretty sensitive from what I hear.
A pair of clownfish (by themselves) would work. Saltwater aquariums can be pretty expensive to set up so make sure you have a budget.
A musk turtle would have to be by itself, I think 20g might be big enough for one but a bigger tank is always better. Make sure you have a platform for it to be able to breathe air and rest out of water.
20g isn't big enough for convicts but you can try a type of dwarf cichlid. Shell dwellers are fun to watch and will breed in your tank. Bolivian rams are cute and not too sensitive.
Tetras come in every colour of the rainbow, I'm sure there is a species that you will love.
If you are looking for personality I reccomend a female betta (you can probably add a few tankmates with her). Every female betta I have had has been full of personality, she will greet you by the tank, follow your hand, beg for food. Great fish all around.