I have CO2 system of Sera, the CO2 tank is 8 litter which I bough very cheap
from Oxar.
The diffuser is of a kind that need water flow, what happen is that it's
getting dirty very often, so I took the CO2 hose and inserted it to the
external filter inlet with an ordinary aeration stone.

It is working so great I had to lower the CO2 flow.