A really large freshwater Hydra?
That is alarming. Maybe a bunch of water changes.
What chemicals have you been using?
Maybe Anchor worms? google those. I had them in my pond a couple years ago, they attach to the fish.
A couple big pwc.
But also is your tap water the same pH?
I think you should add calcium to get the water level to a better pH for GF.
I have low KH and need to add Calcium regularly. A few months ago I hadn't added the calcium in a while I had a sick looking fish, added the calcium and literally the next day fish was doing great and his scales were perfect. The correct pH is important for the fish to be able to maintain its body in the type of water it is in.
Osmoregulation I believe so if the water is in good parameter for the fish, the fish's system is functioning properly and has better ability to keep healthy/fight off illness.
I would add a little salt too if you haven't done that yet. Maybe a 1/4 tsp per gallon, dissolved first, then added. Help the slime coat.
Then the Calcium. I use Kent's Marine dkH superbuffer. Works fine with FW too. As directed. Easy to use just add to tank in water flow.
More recently started using Seachem Replenish, it seems to be doing a great job. I still will add a little of the Kent's now and again as the kH /alkalinity, gets low. scroll down to read the description of how it works.
https://www.seachem.com/replenish.php
I guess there is no end in sight for the miracles of water additives - just found this one. Don't know anything about it or how to use it but perhaps worth checking on!
https://www.seachem.com/gold-buffer.php
https://www.seachem.com/gold-trace.php