I might be a little late but I noticed that a lot of people are saying your tank is way too small, which it is, but not really explaining why. A tank being too "small" doesn't just have to do with the concept of if the fish are physically small enough to have room to swim inside the tank. Obviously that matters, but it's only a small part. The other, and most important part, has to do with the concept of bioload.

Bioload means the amount of waste that a tank can handle before the water becomes toxic to the fish due to a buildup of waste caused by urine and feces. This waste turns into ammonia and then later to other chemicals called nitrates and nitrites. ALL of these are toxic to fish and can cause a ton of health problems, limit the life of your fish, and subdue their behavior. Basically, when tank is "too small" the fish are creating so much waste that the water becomes instantly toxic. I believe this is what is happening in your tank. With a larger tank, the waste dissipates more and does not build up as quickly. With a tank as small as yours with all those fish in them, you'd likely have to be doing water changes nearly constantly in order to keep them from getting sick from all the waste buildup.

It's important to do research on the fish and see 1) How big they will get when full grown. Bala sharks get over a foot long. It's irresponsible to assume you can just give them away when you no longer have room for them. They're living creatures and not accessories. It's also important to consider 2) How much room do the fish need to be truly happy? For example, neon tetras are tiny, but are often recommended to have at least 20gal because they are active and need a lot of space in order to exhibit natural behavior. Sure they could "fit" and probably live for a while in a tiny tank, but it would not be right to do that to an animal that needs space to truly carry out its natural behavior. Also think about 3) How much waste do these fish produce? Some poop more than others and you need to know this because if you have a heavy pooper, it will tax your tank's bioload even though you have less fish. Check out the website aqadvisor.com. It gives you great estimates of what kinds of fish and how many you can safely fit in your tank. Also consider 4) How many of these fish are appropriate to have together? Some fish will be aggressive to any of their own kind. Some are schooling fish like tetra or cories and will become subdued/stressed if you don't have enough of a smiliar kind. Usually 6 is the bare minimum.

I know it's disappointing to hear negative feedback but I think everyone here genuinely wants to help and it's clear you want to do the right thing for your fish! I know my post is long but I hope it gave you some good info to help your fish and I hope they are able to make a full recovery in an appropriate environment.