Substrate - CO2 & Filter Questions

Hi:

I have three tanks that are 36 X 12 X 18 inches and have had some
success growing plants in black gravel and using under gravel filters.
I do not vacuum the gravel since I don't want to disturb the plant
growth which covers almost all of the bottoms, but I do make regular
partial water changes.

I am now looking at buying a bigger tank 48 X 18 X 21 and was told at
the LFS to not use an under gravel filter if I wasn't prepared to
strip down the tank every six months.

Instead, an Aquarium Pharmaceuticals Rena Filstar XP canister filter
was recommended. I have a Rena air pump and am quite happy with it.
How about the filter, is it any good?

If I do go with a canister filter, what should I use as a substrate?
It costs $30 Canadian for a 7 kg bag of Seachem Fluorite. How many
bags would I need and are there any effective alternatives to this
substrate that don't cost as much?


Finally, I have a number of Nutrafin CO2 Natural Plant System
canisters. Is there an alternative to buying the yeast and stabilizer
packets? I don't know what the stabilizer is comprised of. Does
anybody know?

Thanks,

Sharp Tooth