Your water is cloudy because you are WAAAY overstocked.
Lets look at your fish.
-Oscar. Will reach 16". 75G minimum.
-Catfish. I dont know of any cat that can fit in a 7G.
-Koi. Will reach 36" in length, and isn't really suited for home aquariums. 200G minimum is kept in a tank.
-Pleco. No pleco should be kept in a 7G. The smallest one will reach 5", but most wil leasily exceed 12"+. What do you have?
-Tiger Balm? Tiger Barb
? They should be in groups and have at least 24" of swimming space.
-Guppy should be in groups.
Your fish are living in hell. Even two water changes a month will never be enough.
Get all of the fish out.