Tetra Damage: What's Causing It?

I know illnesses spread quickly especially in a small aquarium like a 20. I don't know how many fish u have or how many u could keep in a 20. But u should find out to make sure ur not over stocked. Also make sure u r doing frequent water changes. I have a 55 with 10 medium/large african cichlids and atleast 30 baby cichlids in it and I take 15 gallons out every week so u would probably change 5 or 6 gallons per week. And I use stress coat and stress zyme with every water change. Also you are probably already doing this but make sure to use aquarium salt. That will help keep ur fish healthy too. Sorry I'm not very familiar with ur fish. This is just general fish advice.