You might want to invest in a paintball co2 setup. It's cheaper than a full size co2 setup and more expensive than your ista co2 kit, but in the long run it'll work out better.
My co2 kit is: 20oz paint ball tank with the aquatek mini co2 regulator (Amazon.com : AQUATEK CO2 Regulator Mini : Paintball Regulator : Pet Supplies)
how many beats per second are you running your co2? i have mine running about 1.5bps with light on at 8am and off at 4pm
When you plant your HC, make sure to break them apart so that they're not in clumps. That way they have a higher chance of spreading