What is wrong with my tsn

If you test your water for anything you should definitely test for Nitrates, regardless of water changes. You never know the quality of the water your putting in. I've had it happen that water straight from the tap was bad, could be due to work on the line I couldn't see or a host of other reasons. Point is if you do test it's best to test all the parameters.

I also can't see much of the TSN except his grill/face so hard to see if there's any physical cues to the problem. More info and pics may help in determining whats happening. I had something that sounds similar happen to my Centrodoras just a couple of days ago. He seemed to be being pulled to the surface by his belly. It first started as him leaning to one side slightly and progressed into the more severe case of him fighting to stay down pulled up by his belly so his head was down but his tail was curled at the surface in order to stay in the water, like an upside down J. I deduced the cause and with some confirmation for treatment from TBTB and Aquanero. The cause was over feeding, it caused his stomach and intestinal tract to become engorged as well as constipation and it pressed on his swim bladder making it nearly impossible for him to control his ability to sink or rise. The solution was to add Epsom salt to the water at a concentration of 1 teaspoon per 5 gallons of water or if that didn't work to up the dose up to a max of 3 teaspoons per 5 gallons. First I tried a dip, put the cat in 5 gallon bucket with the suggested concentrations for 30 minutes or so to see if it alleviated the issue. When in my case it still didn't work I added the Epsom at the suggested concentration to a sick tank. Within 24 hours my Centro was right as rain again and I added him back to his tank and within 48 hours he was eating and acting as though nothing happened.

Above and from the side if possible. This would allow me to see how much the belly is distended and if it's more distended on one side then the other which would indicate the swim bladder is being crushed. Sounds like a good plan setting up the 20 just remember to add the Epsom slowly and to start at the 1 teaspoon per 5 gallons, don't jump the gun and watch to see how stressed he gets by it. Another suggestion is putting in driftwood or something for him to squeeze under. This would help him to stay down if he squeezes under it and prevent too much stress from panicking while trying to stay down and from exhaustion.

Oh and I forgot you should feed very very lightly so you can see if he is willing to take food. If he is you can also treat the pellets or food with the Espom salt solution and it'll be at least if not more effective then a bath. I'd still suggest the sick tank though because it's hard to feed light in a community and the isolation will also help decrease stress.