Liquid co2 is NOT actual co2, but a liquid carbon Glutaraldehyde based product. It's still effective, but in the words of Seachem, where (actual, gas) CO2 would be a 10 on a scale of 1 to 10 for effectiveness, Glutaraldehyde (i.e. excel, API co2 booster, or metricide 14), would be a 6 on that same scale. However, Glutaraldehyde has the side benefit of being an algaecide. So by you getting algae, you're either not dosing it correctly, running lights too long, not enough circulation, and/or bottoming out on proper nutrients. You're just not balancing things right.... u know what kind of algae? You can spot treat and do a blackout to try to reset things before you start implementing a 'balanced' approach to your growth cycles.