You should test and post all of your water parameters here.
If you haven't done a water change recently, start prepping some water now. Mix your salt for at least a day.
Have you kept up maintenance with your canister filter? You might want to rinse it well, using old tank water (after a water change) if you want to preserve more beneficial bacteria.
Only feed brine if it is enriched. It will say this on the packaging, and the ingredients will say more that just brine. Otherwise, toss it in the garbage.
I'd buy some PE mysis (spend the extra few $), and a couple of different frozen preparations marked for omnivores.
If you are feeding pellet, use a better brand such as New Life Spectrum.
I'd also soak food overnight, at least a few times a week. Selcon, Zoe+Zoecon, or similar. Shot glasses work very well for this. Freeze dried foods (including brine) soak up supplements readily.
Feed 2-3 times a day, and keep it varied.
All of this can help reduce stress, and boost his immune system.