Thanks for the update — it’s great to hear that Chloe is doing much better!
That’s a very interesting experience you had with Chloe and my best guess is that Chloe was oxygen deprived and very likely in the process of asphyxiating, and that you were subsequently able to resuscitate her by promptly correcting the specific gravity from 1.027 to 1.022. I think your quick action and adjusting the salinity of the aquarium water probably saved her.
You mentioned that Chloe was white when you found her and bobbing in the current coming from the powerhead, apparently not breathing. Seahorses will blanch when subjected to hypoxic conditions, so Chloe’s unnatural pallor was quite consistent with the seahorse that was starved for oxygen, and I suspect she was hanging onto the powerhead with her head bobbing in the current because the oxygen levels in your aquarium were higher where the water was briskly circulating through the powerhead.
The warmer the water is, the less dissolved oxygen it can hold, and you mentioned that you had removed the lid of the aquarium to help cool enough due to the unseasonably warm weather. I suspect that the oxygen levels in your aquarium became depleted overnight and that Chloe was literally suffocating as a result.
Seahorses are more vulnerable to low O2/high CO2 levels than most fishes because of their primitive gills. Unlike most teleost (bony) fishes, which have their gills arranged in sheaves like the pages of a book, seahorses have rudimentary gill arches with small powder-puff type gill filaments. Seahorses are said to have "tufted" gills because they appear to be hemispherical clumps of tissue on stems. Their unique, lobed gill filaments (lophobranchs) are arranged in grape-like clusters and have fewer lamellae than other teleost fishes. Because of the difference in the structure and efficiency of their gills, seahorses are thus especially vulnerable to low oxygen levels and asphyxia.
You discovered Chloe’s plight in the morning, which is when the dissolved oxygen levels in the aquarium are the lowest after the aquarium lights have been off all night. This is because while they are photosynthesizing during the day, seagrass and algae consume CO2 and produce O2, but at night, in the absence of light, this process is reversed and the plants in effect consume O2 and give off CO2.
Finally, there is an inverse relationship between salinity and dissolved oxygen. The higher the specific gravity or salinity, the less dissolved oxygen the water can hold. By the same token, the lower the salinity or specific gravity, the more dissolved oxygen the water can hold. So when the salinity of the aquarium water was too high (specific gravity = 1.027), there was less dissolved oxygen in your seahorse tank than normal, and when you lowered the salinity or specific gravity to 1.022, you increased the amount of dissolved oxygen in the aquarium water.
In short, Jan, I think a combination of several factors combined to lower the dissolved oxygen levels in your seahorse tank to dangerous levels, and that Chloe was deprived of oxygen and that she subsequently blanched or turned white as a result. The oxygen levels in your aquarium dropped overnight as usual, the unseasonably warm weather reduce the amount of dissolved oxygen the water can hold, and the high specific gravity further depleted the oxygen levels (the presence of dissolved salts limits the amount of oxygen that can dissolve in water). By removing the hood of the aquarium to cool down the seahorse tank, lowering the specific gravity so that the water could hold more dissolved oxygen, and turning on the aquarium light in the morning so that photosynthesis could again take place and the algae in the aquarium could then give off oxygen, you corrected the problem and averted a crisis. Once Chloe was getting enough oxygen again, she colored up, her breathing returned to normal, and she had a healthy appetite again.
If my speculation is correct, then Chloe should be fine now that you have brought the dissolved oxygen levels back up to normal. Continue to monitor Chloe while you keep the aquarium cool and maintain the specific gravity at 1.022 just as you have been doing.
The bubble stream from the sponge filter should be helpful, but as an added precaution, I would like you to consider adding one or more ordinary airstones to your tank, anchored just beneath the surface of the water. That will add surface agitation, extra aeration, and better gas exchange at the air/water interface, which will assure that the dissolved gases in your aquarium are in balance with the ambient air pressure. That way, if the oxygen level is too low, the airstones will help raise it back to normal again. But at the same time, if any of the gases in your seahorse tank are supersaturated, the airstones will actually lower the levels of those gases back down to the partial pressure in the atmosphere and restore equilibrium, thereby helping to prevent problems with GBD.
I think you’re on the right track now, Jan, and hopefully Chloe will be feeling like her old self again very soon.