A macro tank in itself is not necessarily something that will suit the needs of seahorses, and needs to be more mature than 3 weeks anyway.
I applaud you for holding off on this purchase for now.
Minimum tank size for standard sized seahorses is 29/30g for one pair, with an additional 15g needed for each additional pair.
Best chances of success would be with over filtered, very much over skimmed tanks, with more frequent water changes and cleaning than for a reef tank.
This helps limit the damage and deaths caused by their affinity for bacterial infections, especially if you keep the temperatures between 68° and 74°F.