Hey hope you are well, new to seahorses and doing my research but can’t find much on Filteration apart from flow not to be strong, my option for Filteration would be internal what would your maximum flow rate be for a 250L tank.
I have 25x turnover flow in my seahorse tank. Its a myth that they don't like flow, they do like to hang in the flow (they even play in it sometimes), it just can't be too strong that they can't swim.
I have an oversized skimmer on my seahorse tank, and I do weekly water changes.
But I've only had them for 2 months, so I'm still learning and still working on refining my husbandry habits.