As already mentioned, the 20g tanks are TOO SMALL for standard seahorses available to us, and TOO LARGE for dwarf seahorses unless you are going to waste a pile of live enriched baby brine shrimp every day.
Also, you would be hard pressed to even SEE the dwarfs in a 20g tank unless you had 50 to 100 of them in there. You could fit a probably about 6 to 8 adult dwarfs on a silver dollar.
Just remember also that keeping grasses and macros are going to require a fair bit of light and that usually means heat added to tanks that should be kept in the 68° to 74° F range for best chances of success.
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