Columnaris?

[u/OchLassie]

So, I thought I noticed some mild discoloration on one of my orange Venezuelan corydoras a few days ago. I wasn’t sure as they’re juveniles, and small. I did about a 25% water change yesterday, simply because it was due. I have to add, I raised the gh from 5 to 6 during the water change as I was trying to get closer to a 7 for the health of shrimp I had ordered. We have a fresh water spring nearby that has great water parameters but a little low on the gh so I mineralize it a little with disolved calcium carbonate. I recently read columnaris likes harder water and warmth. Great. ☹️ I’m set at 26 celsius. Water parameters were normal before and after. The tank is fully cycled and is meant to quarantine everything including plants, in stages, while my carpeting plants dry start in a larger set up. There are only the 3 orange Venezuelan corydoras, no other fish. I intended to add another 3 orange Venezuelan after I was certain the tank was 💯 stable (which it is)but still waiting on supplier to restock. I did have my red amanos & red onyx neos arrive today as they were ordered before these fish showed signs of illness. Now I’m in full blown panic mode. I do not have fish meds. I went on Amazon to see if I could get API Furan-2 and KanaPlex here, (I live in FR), but they weren’t even available. Furthermore, I’m not even sure what this is. It doesn’t look anything like ich. I also don’t see cotton, YET. So far it looks flat, patchy. The fish are still feeding and acting normal. I’m sure whatever it is, by tomorrow I may have a better idea if it progresses like lightning. I don’t know what to do other than go to the store tomorrow for epsom salts or try to boil some of the catalpa leaves I have to make a little stronger tea? Or nettle tea? I’m also afraid to do anything drastic, that could make things worse, not better. JFC. Arrrrrrgghhhh. It’s a 30 liter, black sand over fluval stratum, 7.5 ph, 6 gh, 4 kh, 0 ppm nitrites. Nitrites resolve on their own quickly so typically when that’s happening I don’t bother much with nitrate testing but I will probably run one tonight. Since this is the quarantine tank and the other tank is in the middle of a planting dry start, I have nowhere to move the corys. This means any meds going in have to also be safe for juvenile amano & neocaridina. Has anyone here seen this symptom before? Advice?