Would your parents let you/buy you any meds to try with him? I'm wondering whether an antibiotic med, or wormers, might be the solution, or at least worth trying since salt hasn't helped so far and he's getting worse, and there isn't any obvious signs of external infections, fungus or fin rot, and the water parameters are good.
I'm suspecting some type of internal parasite or bacterial infection, since he started off with tail drooping, but was still eating and swimming okay, and that the infection or worm load has become bad enough to start affecting his swim bladder. But that's just a hypothesis based on very little real knowledge or experience, since I'm a sorta beginner. Have some personal experience with battling different diseases, or seeing them dealt with in my parents tank as I was growing up. I wouldn't offer this advice if someone with more knowledge was here and able to advise, but if someone who knows more gives you different ideas, please listen to them over me! I Just want to try and help since I've been in your shoes and know how upsetting is feeling bad for them and helpless to fix it.
My current working model of fish illnesses is basically broken down into;
1. Parasites, external or internal - e.g: roundworms
2. Fungal - white fluffy patches, usually easily solved with salt treatment
3. Bacterial
So first we look for water quality issues, and yours is fine. Then physical symptoms, to narrow things down, and yours isn't showing outer physical damage of any kind like in fin rot, ich, flukes etc. Then we look at behavioural clues, and yours isn't fin clamped, refusing to eat, gasping at the surface. But he was fin drooped and maybe lost some colour? And is eating. Was swimming okay, then more affected, then it got worse despite salt treatment, so it seems to be something internal, and it's worstening enough that it sounds like it's affecting his swim bladder. Most common and main suspects in my mind are a bacterial infection, or an internal parasite like worms.
If it were my fish, and I wasn't able to get further info or help, I would try an antibiotic treatment first and see if it helps. If it doesn't, I would try treating for worms. When my guppies were showing symptoms of worms (drooping tail, getting skinny but eating well and no other signs of illness) I went for broad treatment and I'm using two meds for worms. One for flatworms, and one for roundworms. I'm also using another med that is anti-bacterial. I've not been able to narrow down what it was affecting them, but it seems to have worked, and they're looking much better now. I don't like just throwing meds at fish at the first sign of trouble, or without knowing precisely what we're treating, but I also cannot just sit back and watch a fish suffer without even trying some meds, even if it's a last resort.
I'm afraid I don't know what meds are available in the US or which are the most effective and safest. Be aware that an antibiotic treatment could also kill off a lot of the beneficial bacteria in your tank, mine did cause a mini cycle in my tanks, but I have a lot of live plants and did water changes, and the BB bounced back within a day or two. So you'd need to test the water daily and be prepared to water change/re-dose meds, and ride out a mini cycle if you use one. Maybe
@Wills has some better ideas, or can give you info on possible meds that might be worth trying?
I will double check on the Petsmart website though.
