[TamiW]

It usually takes a while for the bad things to catch up with your tank. It works fine for a while, so you assume it can't be the cause - but the reality is, when people give recommendations for best practices, it doesn't mean that problems will arrise immediately if you break the rules - but somewhere down the road, you likely will face those problems.

I did a brief stint working in a fish store several years back and I saw this all the time - someone was doing something they had been told not to, and it worked for a while sometimes YEARS and then things would quickly go downhill. Because the thing they had been doing for years appeared to "work", they'd argue vehemently that it couldn't be said factor. Water changes were the most common, people deciding not to do them at all, then wondering why 2 years later everything was going wrong.

That's probably what is happening with your tank now. That phosphate level is pretty high. Phosphates won't harm your seahorses, but it is a sign that your tank is producing a lot of organic wastes. I'd like to see a complete break down of your water perimeters in addition to what you've got now - nitrite, kh and calcium. But regardless, I am going to lean towards a combination of temperature and water quality. Gas Bubble Disease in seahorses is almost exclusively an environmental issue - the specifics can vary, as you can see from the discussion between RayJay and myself - but the short version that it is believed to be almost entirely caused by environmental factors.

I'm not suggesting your tank is a terrible place, but seahorses are more sensitive than other fish so even something just a little out of whack might be too much. Also, if your seahorses were considerably smaller when you bought them, they might have been fine; but growing in the subsequent months they now make up a much larger biomass of the tank.

You are right that moving to a new tank will be stressful, but if they were mine, and I had a cycled tank ready, I'd choose to move them and reduce the stress of a too small tank over the stress of moving them. I've kept seahorses in 10 gallon tanks temporarily, but the key is it was temporary, and all but perhaps the smallest species will be cramped in a tank that small for any prolonged period of time.