Usually once you put fresh garlic juice on the food they will eat it better, I make my own fish food, but you can look on the fish food containers and see what it list for the protein, look for at least 43-50% or higher.
I use fresh garlic that I grow and I use a garlic press on it so it is nearly a paste already and lots of juice comes with it, the pre-chopped/crushed jarred garlic you buy for cooking can go bad if your not careful and sometimes will be oily.
You can buy the garlic booster for fish but I have never used it myself so I can't comment on its effectiveness.
It is important that you use the right dosage when using aquarium salt or any medications for that matter, otherwise, it may not be as effective and it can cause resistant pathogens/bacteria...etc......and you create super bugs so-to-speak........that is why I recommend that you pre-mix in 1gal jugs and do 100% water changes to insure correct dosage.
Sometimes frequent water changes are all that needs to be done when you catch issues early, I found over the years anytime my fish are acting "off" I will make a water change of at least 50% and that usually perks them up...even if I had just done a water change...
I can't stress enough the importance of water changes, this is their environment, just like us when we are in a polluted room with poor air quality...fresh air makes us feel better.......