I'm sorry if I sounded abrasive, I did not mean to.
This is what I would do. Take into consideration that I am not familiar with what equipment you have available in your country. Seahorses look great against a black substrate, but they are more work to keep clean and nice looking. You are aware they cannot be kept with any stinging invertebrates? Get a good refractometer. Will you be making the water for the tank yourself, or purchasing it? I would add a sump, to place equipment,add water volume, and add oxygenation to the tank. Keep your heater and protein skimmer here. Will the tank be drilled for an overflow? Perhaps consider growing seagrasses and macro-algaes first. Put sand, live rock, and cycle the tank for several weeks. Some macro algaes such as the red colored ones can be touchy in a new set up. They can be quite beautiful if set up properly. Consider that displacement from rock & sand takes away some of litres available for your ponies. . I would also set up or at least have available, a quarantine tank. You might want to consider tank bred horses over wild caught. The price is much higher, but mortality rate is lower...as they are accustomed to life in an aquarium. They don't come with the various maladies that seahorses are prone to, and are raised to eat frozen mysis shrimp. Get a good quality salt mix. With a marine tank, patience and research are paramount and will bring you success. I have not personally kept seahorses, other than working with them. They are very beautiful and fascinating. They are not an easy first marine fish to keep. I would add two pairs to start.I will help any way that I can.Good luck. 8)