Sounds like a parasite or bacteria most likely internal parasite. I had the same problem due to nematodes. Hair thin white worms mistaken for planaria. I asked a buddy at the LFS & took a water sample w/ the worms and that's what it was.
You can try parasite clear made by Jungle, but didn't get good results. So i asked Mike on what to do. I had to start all over complete break of everything boiling gravel, decors complete tank including filtering systems for an 1-2 hours then treat with Mardel Copper Safe liquid meds good for a months time on one solo treatment.
It kills internal & external parasites. Not recommended for inverts or snails. Kills everything in the aquarium but the fish. No over dosing on Mardel otherwise you'd poison your fish killing them as well. I did take advice in cutting down on feeding from a guy on here a whole two days went by and now both my plecos are dead/eaten up to the head by my piranhas.