What is the minimum size tank for a saltwater aquarium?

Seahorses as their name implies are marine creatures. The basic rules for a marine tank are 1 inch of fish needs a minimum of 5 gallons of water. :- Every tank needs a permanently running cycled filter. Every tank must have an efficient permanently running protein skimmer. The bigger the tank, the easier it is to keep stable. I would never consider trying to keep any marine fish in a tank of less than 50 gallons.