Seahorses.

[rayjay]

Yes, a 20g is too big for any dwarf tank unless you are wanting to keep about 100 or more of them, and it's too small to keep any standard sized seahorses that we see here in Canada.
Dwarf seahorses would get lost in a 20 because of their size. You could put a few on a loonie.
The REAL reason though its because of their feeding habit, which is to MOSTLY sit perched on a hitch and wait for their food to pass by close enough to snick without leaving the hitch. While some may hunt a little food, it won't be enough for them to survive.
This means that the density of enriched baby brine shrimp in the tank must be very high, and, then it needs to be removed one way or another before the next feeding, so the amount of waste would be extreme.
If you DON'T remove the food, they end up snicking nutrient poor brine that again can lead to shortening their already short life span.
Dwarfs on average live about a year and a half, whereas their larger cousins can be living 5 to even 10 yrs if conditions are kept ideal for them.