I think much of your CO2 is leaving the tank as fast as it comes in, but with no more details it's hard to be sure. Is the canister filter outlet near the bottom of the tank? Where is the filter intake relative to the outlet? What plants do you have and how densely planted is the tank?
Valid points there Hoppy. The spray bar (outlet) is in the middle of the tank & I have placed it slanted horizontally (at the back glass) so starts off 2 inches below the surface & ends up 8 inches below from the other side. The filter intake is located in the bottom right side of the tank. I have somewhat average surface agitation created by the powerhead mostly at the left side of the tank.
My plants are: Blyxa Japonica, Staurogyne, Echinodorus Quadricostatus, Ludwigia Repens, Cabombas, Java moss, Java Fern, Hygrophila Polysperma, and duckweed. I'd say the tank is about 90% planted.
Cabombas, Hygro's, duckweed, Ludwigias, Echinodorus Q. are growing fast.
Java fern, Blyxa, Staurogyne are growing slow but theyre doing fine.
There is some BBA on Java moss & Echinodorus Q.
The older bottom leaves of Ludwigia Repens & Hygrophila have pinholes in them & eventually fall off (specially in Ludwigia).
Some green spot algae in the bottom side of the front glass.
I used to have Rotala green but I pulled them out coz they didn't do too well in the tank.
I have 3 X 54 watts t5HO bulbs that run for 8.5 hrs a day. Theyre one year old so I don't think they have same intensity as before.
temp. 79 F
ph. 7.2 average
nitrate 20-30 ppm
co2 god knows
Ferts: EI method although I dose less N & more K instead.
I dont dose GH booster