Hi Oz;

You really should keep track of your pH, at least, when you start injecting
CO2 ... you don't want it to drop too low.

I'd recommended getting a pH kit, and if you can, a KH kit as well so you
know how much CO2 you're getting in your tank. Too much CO2 can also be
lethal to your fish.

Once you know your pH and KH level, you can determine your CO2 level with
the chart or calculator at:

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm