PLEASE, don't use a fish to cycle your tank as there is ABSOLUTELY no need for it. In addition, it is possible that you may add a pathogen(s) to your system that can affect the future seahorses you place in the tank.
All you need for the cycle is an ammonia source, which can be ammonium chloride, liquid ammonia from the grocer (without any additives or perfumes), or just food that will decay and produce the ammonia.
Some use bacteria products added to the tank to speed things up but I've never done that myself.
Please remember to clean the pads in the filter AT LEAST once a week, more frequently is better, so nasty bacteria like the vibrio species don't get colonized in any big way.
All you need for the cycle is an ammonia source, which can be ammonium chloride, liquid ammonia from the grocer (without any additives or perfumes), or just food that will decay and produce the ammonia.
Some use bacteria products added to the tank to speed things up but I've never done that myself.
Please remember to clean the pads in the filter AT LEAST once a week, more frequently is better, so nasty bacteria like the vibrio species don't get colonized in any big way.