Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I would use the smaller pump as part of the actual diffuser system, but maybe add more water circulation nearby. When the water (rich with CO2) comes out of the diffuser it may need some help to fully circulate in your large tank.
The UV might help with this, but I would not put the intake very close to the CO2 enriched water. Put the outlet nearby to enhance the circulation.
Think of all the pumps (power heads, filters, UV...) in your tank.
Make sure they all help each other, all flow in the same direction. If you have one that is aimed against another all you are creating is turbulence, no a good flow.