Well, even if the fish are small it is a huge stocking issue that you have. The bioload of the goldfish may just be too much. Also, you should only have 1 rainbow shark per tank, no more. Multiple rainbow sharks can result in territory issues (this rule also applies to redtail sharks). A Bala shark needs AT LEAST 50G to start and when it gets bigger it could use as much as 100G of space. Balas can grow very big, and the small size of your tank may be stressing it out, because they need a lot of swim room. The goldfish are pigs, and they will eat everything before it can get to the Bala too, so to fix that issue you might want to try putting food on both sides of the tank, or whatever works so your Bala can get to the food before the goldfish. The goldfish will keep eating even when full
so it might be tricky at first.
Aside from stocking issues, you also have the COLD VS TROPICAL issue. Goldfish are coldwater fish, they prefer much lower temperatures. All your other fish are tropical, they prefer temps btw 68-82F... the Goldfish will prefer temps below 68F....
Sadly if you want these fish to live to their full potential, you cant keep them together because of the temperature needs initially. Goldfish also need lots of tank space because of their bioload (15G per goldfish is the rule). So 2 in a 20 is pushing it already.
I'm 90% sure the rainbows should not be kept with their own species, and other bottom feeder sharks. 1 rainbow would do okay though with a school of Balas.
The Bala shark would do much better in schools and they have potential to grow >>> a foot long, that is why large tanks are recommended.
This is the downside to putting small fish that grow in small tanks; you hamper their growth rate and they dont grow properly, and die prematurely... or they just die. In some cases they might just GROW, and then be too big for the tank. It's your fish you make the call
You'll be fine for a year or so maybe, but I would make room for a larger tank sometime soon, or reconsider what kind of fish you keep. There are lots of fish that can be housed in the 20. None of the ones you have except the Cory are really meant for 20G tank. GL and welcome to the tank. You will get tons more advice throughout the week