.5% salt is quite a large amount of salt. I can not see the injured fin very well in the pics. You mentioned that it looks cottony- does it look like strands or tufts of hair/fuzz growing out or more of a white, rough appearing film? This will help to figure out if it's fungal or bacterial. Also, where did the fin tear/ how badly?
You can use aquarium salt but limit it to one flat tablespoon per 5 gal
. You will need to premix it in a cup of tank water & add it gradually over a 24hr period. When you do water changes, add back only what you remove (ie 50% wc
, add back 1/2 original dose). I would also up your water changes to 50% to provide him with very healthy water while he heals. Make sure you temperature match & properly condition the new water.
Once you have a test kit, you will be able to figure out a water change schedule based on your numbers. Please do not hesitate to ask questions!