Just wanted to give an update.
Purplius is SO much better! I treated him in just my 2.5 gallon rectangular tank, but at first I really only filled half of it because he was breathing so hard I wanted to make it very easy for him to reach the top. I put just a leaf hammock and a little place to hide (small cave thing), treated with salt and the only all-in-one treatment they had at my pet store. It seems to have worked. I did water changes every day. I cleaned out his 6.6 tank very well, cleaned the filter, kept the biological medium, and moved his tankmate to another tank of just platies. I also treat the water with peat granules because our water is so hard/alkaline. I put a little stress coat in his water also. I gave him a variety of foods as usual, and I've picked up another brand of pellets to add to the rotation. I did not get the Hikari because as someone has mentioned on another thread, they may have changed their formula. I chose the pellets based on whether soy, corn, or wheat is the main ingredient. I prefer fish or worms or insects to be the first few ingredients, not fillers. Hikari had more fillers than Omega-whatever.
The other day I put him back in his new tank after I tested the ammonia, nitrate, and nitrite. He looks SO happy and energetic. His gill still looks a little off, like someone here said it would, but I think he is going to be ok.
Thanks everyone, for your help. :) I will post pictures soon if my phone camera stops acting up.