Symptoms:
I noticed some sores on the belly of one of my hillstream loaches (spotted). Lets just say that my google searches have only worried me more and I want to get an opinion from the community. So far this is the only loach I have seen with any kind of sore in my tank. I think I technically saw one sore a long time ago but thought it was just a minor injury that needed time to heal, it didnt look that bad. I hadnt really noticed it since because this particular loach has always been shy and doesnt really like to hang out on the glass. I have her (?) in separated in a floating breeder tank for observation, she was unbelievably easy to catch today, almost concerningly so (she wasnt just sitting still but she wasnt putting in that hillstream-loach effort into evading capture).
Tank info:
~My tank is a year old, 30gal, temp between 70 and 73, airstone and surface agitation with plenty of filtration (cascade 1000)
~Parameters are 0, 0, ~20 nitrates.
~I have 6 hillstream loaches (2 spotted, 4 ringed), and 15 rosy loaches. I have an uncounted number of rili shrimp, an uncounted number of young ramshorns, and an uncounted number of assassin snails.
Decor:
~Rocks:
~~~A number of very smooth stones,
~~~one stone that is not the smoothest but also not too rough (to put in context, at most the coarseness of medium grade sand paper). Note, everyone in my tank hangs out on this rock so id expect the sores to be on everyone if it was the problem.
~~~substrate is a combination of fine sand and semi-rounded gravel (not even close to sharp).
~Wood:
~~~A few pieces of driftwood and some thick bark.
~Plants:
~~~Giant hairgrass, moss, glosso, Elodea, hornwort.
What is it, and what do I do?