you use a outside tap

you dont use
RO water

? i would use ro and if possible
RO/DI water this will save you losts of money in dead fish and inverts and corals if you planning on keeping them ,, as fresh tap water has a lot ov copper traces and corals and inverts are copper intollerant , also tap water has a lot of
phosphate and chemicals in there that fish and inverts dont like and will cause you to have a alege problems to ,
seahorses your right they dont compete for food and would starve and they are very hard to keep and tring to catch them everyday would kill them even quicker ,
as for your first fish i would buy a chromis and keep for a while in a
QT tank as if you introduce whitespot in to the tank you will have problems qt all fish before putting in display tank will save you a lot ov money in the end ,,