06-26-2013, 11:32 PM
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Turning off the bubbler will help. As JDM said knowing your lighting as well as any fertilization will help in figuring out if you need and would even benefit from CO2. CO2 is not a cure all, it is a single nutrient and usually the last element in a planted tank that will become limiting.
The volcano bubbler and the HOB are both going to actively remove CO2 from the water you would need a true CO2 system to combat using a HOB filter. As JDM also said it will take a lot of yeast and a lot of sugar to make an effect on a 75 gallon, followed by the upkeep that comes with a yeast system. Irregular CO2 levels are just as bad as low CO2 and can encourage algae.
I'm still not sure what you intended to do with the air pump but a yeast CO2 system needs to be tightly sealed. Silicone all connections very well and make sure bottle caps are on tight. The system will slowly generate its own pressure which is used to feed CO2 into the tank.