Well, yes, if the external filter from your old tank includes biological filtration media and was transferred to the new aquarium with its population of beneficial nitrifying bacteria intact and fully functioning, then it should help provide biological filtration for the new seahorse tank and hopefully avoid any ammonia/nitrite spikes during the transition period. Break your test kits out and monitor the ammonia and nitrite levels closely for the first few days to make sure the biofilter is functioning as expected. Be prepared to transfer the seahorses to the in-tank refugium on your 55-gallon tank if the biofiltration doesn’t perform as well as you had hoped. Gotta have a backup plan for your helper in case things go south while you’re on the road…
Yup, it’s an unfortunate fact of life for us aquarists that the fish sitters we rely on when we’re away from home are often not up to the task. It’s been my experience that fish sitters very often have a tendency to overfeed, so I would instruct your helpers to feed the seahorses sparingly while you are away and prepare portions that are perhaps half the size of what you would normally provide. In fact, if you will only be gone for a few days, Nigel, then it might be best to give your seahorses a big meal before you leave and then just allow them to fast until you get back. That way, your fish sitters can just check to make sure your equipment is operating properly, which will minimize the chances that they might mess things up despite their best intentions.
Best of luck making the transition to your new, improved seahorse system, sir! Here’s hoping everything goes smoothly while you are away.