salt and MOST of the fish you have in that tank, will not work..they cant tolerate salt at all....if you can, get a
test kit asap as I still think its something with the water parimeters....the ph you keep talking about, isnt an issue and usually doesnt really matter as long as its stable and doesnt change often...stop worrying about the ph

at this point, you need to do some water changes and if you can , find some prime for the water to neutralize ammonia and nitrites, in case you have them...it will also help with the chemicals in your water....until you get a test kit,
API master liquid is a great one as it has everything you need, id continue with partial water changes everyday ...goodluck