Looks like it may be plugged up, aka constipated. Is the fish eating? This matters in how you might try to treat it. Not all meds work well being dosed in the water, they need to be consumed by the fish.
Constipation does not have a single cause. Bad diet, internal parasites/worms or a bacterial infection can all be a cause. Constipation can be a symptom of something else. I would suggest any treatment be done in a Hospital tank. This should be bare bottom and hopefully contain a few artificial plants to make the fish feel more comfortable. A bare bottom will allow you to see if and what the fish is pooping.
The tank need not be cycled but you will have to do daily water changes of at least 50%. If you are medicating, then after the water change you must replace the amount of the med by the amount of the water change. That is a 50% water changes means a half dose of the recommended amount should go back into the tank, a 25% change means replace 1/4 of the full dose, etc.. If you are supposed to add a regular dose of the med on that day, then do the full dose after you refill the tank with clean water.
Also, I would put a heater and a cheap airstone or sponge filter in the tank. If you never figure out what was wrong and the fish dies, you will need to bleach everything in the tank and throw out anything that bleach would degrade/break down (pretty much anything organic). Anything bleached which will be reused should be rinsed and then allowed to dry out 100% to make it safe.
Since you are keeping clowns and do not have these, here are two links to good informarion about the species and how to keep them in tanks:
https://www.seriouslyfish.com/species/chromobotia-macracanthus/
https://www.loaches.com/species-index/clown-loach-chromobotia-macracanthus
My largest clowns are now in the 10-12 inch range TL and they are still one of my favorite fish.
Edited because the above post re recovery was made before I hit post for what I wrote here. But the fact the fish did recover would suggest it was constipated and that the blockage cleared.