3 tsp /gall of the plain Epsom salt is fine. Usually epsom salt for internal problem. So usually people will recommend aquarium salt. But Epsom salt also has antibacterial affect. So it fine to use it if you think he feeling better. Epsom is very mild and don't stress the fish so you can use it. And you also can always switch to aquarium salt if needed .
If you not going to use the filter you can alternate 50% water change one week and 100% another week, but you need to vacuum the gravel with 50% water changes. I don't know how other people will recommend but i personally do just 100% water changes for my 5 gall. I do it every about 8 days. So if it easier for you you can also do that. And you don't need to vacuum the gravel then.
How i understand that you never did 100% water changes in the main tank right? You just doing 50% water changes. I would do 100% water change tomorrow with 3tsp/gall epsom salt. Then you can continue with Epsom salt if you think it helps . You really can lower the water in the main tank so it easier for you to change the water. And while you doing epsom salt you can alternate 50% and 100% water changes daily. And then if he feels better just go to regular water changes.