I’m sorry to hear about your problem with hair algae. It’s great that your nitrates are very low, but it’s also important to keep your phosphates under control and keep the nutrient loading in the aquarium very low, as well as maintaining the carbonate hardness and pH at the proper levels, when you’re dealing with the dreaded hair algae.
I know that many reef keepers will use fluconazole to eradicate hair algae and that it apparently does no harm to the delicate corals and invertebrates in the reef tanks. And I can tell you that seahorses typically tolerate all of the usual medications and chemotherapeutics designed for use in aquariums very well, just like most other marine fish do.
But I cannot tell you for certain that fluconazole is safe for use with Hippocampus erectus, since that is something I have never tried and therefore have no personal experience with. That might be a good question for you to post on one of the reef-related discussion forums, Tanya.
When the hair algae begins to die off, be careful to siphon out as much of it as possible so that the dead and dying hair algae does not release the nutrients it’s been absorbing back into the aquarium, and so that the decomposition of the dead algae does not deplete the oxygen levels in the aquarium.
To be on the safe side, it might be a good idea to temporarily relocate your seahorses to a hospital tank or quarantine tank while you eradicate the hair algae. Stay on top of the water quality in their temporary corral with frequent partial water changes, as needed, in the meantime.
Pete Giwojna, Ocean Rider Tech Support