I'm more into the don't wait, do something category. When I find a seahorse that appears in distress, especially if I don't know the reason, I transfer it to a hospital tank with new water matching temperature, pH and specific gravity, making sure it has heavy open ended airline aeration.
IME, heavy breathing is usually due to a stress on the seahorse which can be caused by another tank mate if any, or by a parasite infestation, or a disease that is taking hold and that one sometimes is the hardest one to pin down.
I agree that we need more information about conditions and that it's reasonable to assume the cycle was not complete, or, insufficient for the loading of 3 seahorses and the amount of food you are providing.
If it was my seahorse and I couldn't determine the cause of the problem, AND, if the seahorse was NOT improving, I usually start with a freshwater dip to see if there IS an infestation of parasites, and, if NOT, especially if getting worse, then I would usually do a complete cycle of an antibacterial agent like Furan II that I like to couple with tri-sulpha.
Unfortunately though, it's also possible that an internal situation like liver disease or some other organ problem is present and you are unable to determine this for sure unless edema becomes evident. That would require used of prescription drug Diamox.
If there is a water or food causative for this problem, remember that this one may have a weaker immune system and is only the first to be affected by it and it's possible the others may also succumb to the same problem. If their systems are sufficiently better, they may not come down with any symptoms at all.
If after placing in the hospital tank with the new water, it improves in a day or so, then I'd for sure be looking at potential water problems first, but also, being sure my food wasn't harbouring any nasty bacteria.
When feeding live brine shrimp, at whatever size, they should first be sterilized somewhat by adding hydrogen peroxide to the enrichment water just before feeding them to the seahorses and then rising well. (assuming you ARE doing the recommended enrichment with a high DHA included product)
In the aquaculture industry, artemia cysts (brine shrimp) are WELL know to contain a lot of nasty bacteria that if not properly treated will still be with the artemia when being fed to fish. Seahorses have the GREATEST susceptibility to bacterial problems of any fish I've ever kept over many decades.
What I DON'T agree with is the notion that you need LOW FLOW in a seahorse tank.
That WAS the recommendation 15-16 yrs ago when I started, but it changed within my first few years of keeping to state that it is preferred to have sufficient water motion to be able to keep detritus in the water column long enough for the mechanical filtration to pick it up. (and of course, clean that filtration every 3-4 days BEFORE any serious decay set's in)
I believe most seahorse keepers now are at LEAST in the 10X flow with a good number at 20X+-. The main criteria is to have hitching available in areas of low, medium and high flow so they can choose at any time just what they prefer, and to have no blast that can propel them against anything to damage them.
You will likely find that many seahorses actually seem to "play" in the strong output, returning many times to "go with the flow".
