Hiya kelvin, have you just joined? Hello, if you have
A rough rule for how many fish is 1" per gallon, and that's taking the adult size of the fish.
85litre is about 18UK gallons, so that's 18 adult inches of fish you could take.
I think I'm right in saying that there is a bit of give in this because it does depend on what filtration you have and that, but it gives some idea anyway.