Next time you need flake food, Omega Super Color flakes are a bit better & readily available but Tetra is ok. Others may not be, read the labels (with or without your glasses on, ha!). You want fish, seafoods or plant food like spirulina as the first 3 or so ingredients. Not wheat, rice etc., they're just fillers or binders.
I treat salt like it's a drug, I don't use it unless I know what it's supposed to treat. It can help with "brown blood disease" from high nitrites or as a treatment for ich (not my preferred 1). I almost never use it but table salt is fine if you choose to go there, no need for "aquarium salt". If you use it add it slowly over a few days...& then remove it slowly too.
Water changes are my best advice for now, & a lot of them! Keep a close eye on the fish & tell us what you see on the fish or their behavior. White or red spots or streaks, red gills, flashing or shimmying.