Would a 20 high work?

The general rule of thumb is 29 gallons per each pair of seahorses.
That is not correct tho, if you are talking about dwarf seahorses.
Do you have any idea what species you will be getting?
 
Yes, a 20g tank is not suitable for standard sized seahorses common in the trade.
There are some that say a pair of H. fuscus can be kept in that size of tank but there doesn't appear to be any availability of this species.
One of your sixty fives would handle 6, possibly eight seahorses if husbandry is properly handled.
If you have a sump with it, then possibly even more could be kept.
Recommendation is 29g minimum for one pair with an additional 15g for each additional pair.
 
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