What species do you intend to keep. Each species is a different size.
A 20g is too small according to most people unless you intend to keep smaller seahorse species like capensis, procerus, or breviceps. They all require cool temps below 70F though, so you would need a chiller or fans.
Rotifers are typically only used for fry of reidi and maybe kuda. However, they are both larger species that would need a larger tank than 20g high to breed and thrive. Most fry accept live baby brine shrimp readily. Fry would also need separate tanks from the parents.