Seahorse belongs in a marine (saltwater) aquarium. The fish you have had were all freshwater fish. If you have not had experience with saltwater aquariums, it is not recommended to jump onto having seahorses. Keeping a saltwater aquarium is a lot harder than having a freshwater aquarium. (it is also more expensive).
By the way, the fish you have mentioned don't not even belong in the same aquarium. Betta is tropical fish, goldfish is not. 
So what kind of aquariums have you had? I would recommend to have years of experience with a freshwater tropical aquarium before getting the first saltwater aquarium.
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