A couple of things.
- Is your tank cycled? Do you know your water parameters (pH, ammonia, nitrites, nitrates)?
- A 20 litre tank (5 gallons) isn't big enough and you are keeping fish that should be kept in groups.
Neon tetra minimum number 6 to 8 in a 40 litre (10 gallon).
Platy minumum number 4 in 40 litre (10 gallon).
Black skirt tetra minimum number 5 or 6 in a 40 litre (10 gallon).
Harlequin rasbora minimum number 8 in a 40 litre (10 gallon).
If you wanted to keep all those fish, and keep them in numbers that they will thrive in, you should be looking at a 120 litre (30 gallon) tank.
If you dont want to upgrade your tank, you should look to rehome your fish and settle on fish suitable for a 20 litre. A single betta is the most obvious that springs to mind. Or have you considered shrimp?
If you persist in stocking schooling fish that are unsuitable for such a small tank, they will inevitably be unhealthy and unhappy.