Some use it for maximizing colours in their fish but I believe it also works to enhance the health and well-being of the fish.
Personally, I have Dan's Feed from seahorsesource.com that has Astaxanthan in it but much more.
From Dan's site:
Dans Feed is a blend of AlgaMac 3050, AlgaMac Protein Plus, Spirulina, Astaxanthin, Withania somnifera, Mucuna pruriens. Well balance nutrionally with amino acids, vitamins, minerals, proteins, Omega 3 fatty acids with a very high DHA to EPA ratio. Also contains Ayurvedic herbs to increase resistance to disease without risk of antibiotic resistance, increase food conversion ratios, reduce stress, and in sexual mature seahorses, help induce spawning.
It comes with and without Beta Glucan as you don't want to use the Beta Glucan all the time.
You mix this powder in a blender and feed the artemia/mysis to enrich them so the seahorses get the full benefit.
Dan can mail this product to us here in Canada.