OK. Tank is ~250g, inc. refugium. About 1.5y old, pretty mature. Seahorse partition is about 60g, coral reef partition is about 120g.
Temp is 78 (night) – 79 (day). Salinity is 1.023-1.024. Ammonia, Nitrite ~0. Nitrate ~2-3ppm. Calcium ~400ppm, KH ~125ppm. pH generally around 8.2, but I haven’t measured in a long time.
The real clue is that everything else (stony and LPS corals, fish, seahorses, clams, inverts) is doing great, and only this one seahorse is having trouble. Perhaps water chemistry is an issue, but it can’t be too far out of whack, since tens of other organisms are clearly thriving.
Also, I got peppermint shrimp months ago. May have "helped", but didn’t "cure" my aiptasia problem. Although the one in the seahorse side may have escaped out the overflow and into the refugium somehow. I’m going to have to try to catch it and move it back.