Seahorses are hard to keep due mostly to their feeding requirement, which usually creates difficult water conditions. Nitrate is a problem I constantly fight with with my seahorse tank. You have to feed quantities which allow for the slow, deliberate feeding habits of the species, and this usually means a bio overload, followed by higher nitrates.
The two biggest concerns for water quality with seahorses is nitrate and the ph.
Dwarf seahorses add additional feeding difficulties due to their size. More notably, the smaller size of food, ie..brine shrimp nepulai. My experiece with the dwarfs is they need to be in as small as tank as possible, around 5.5 to 10 gallon max. This would allow for the proper concentration of food for the little guys to consume. A good 20 minutes, 2-3 times a day is needed to allow the animals to consume their fill. I tried them in a 45 and it was a disaster. As you can imagine, removing the excess nepulai after the 20 minutes is impossible.
I currently house the larger species, H.kuda and H.barbouri in the 45 gallon. Even with these guys, the bio load requires tons of attention to maintain the proper water quality.