Wait so you think it really fin rot?
Do not put 1/2 tps of the salt. It not a therapeutic dosage and will not help , and just he will get resistant to the salt , and the salt will not help when needed.
So if you can keep up with water changes, stress coat helps. For 1.1 gall tank i would do 2-50 and one 100% water changes. Monitor him if you really think he is worse then you can add salt. But you either need to use theraputic dosage and full course of treatment or Do not use it at all.
The instruction on the box - just for the preventative care so do not read them.
You saying he started ti show sings after 100% water changes - it probobly just co incident. I don't think full water changes can be the problem. Just make sure you acclimate them to the new water temperature so you don't stress them out.
Tank divided by 3 good luck. I am not fan of that but i know many people not agree with me. I like to keep them separate in case one get sick . I don't want to end up with all of them sick.
But if you buy bigger tank you need twice bigger filter. And make sure it not too strong , then you need to ruffle it up, and you still need weekly water changes and filter media rinse/swish in the tank water.
Fin rot-dark,discolored or ''smudged'' edges