i am starting my planted tank, and i want to work from bottom up. my forst thing i want to work on is the substance. i have read that people have different layers of substance in their tanks. i am low on money so i am using half a cm of the gravel i have now(because it is full of the bacteria i need) and then on top of that i want to put typical play sand. i have read that there is a sort of fertilizer people use. it sounds expensive. is there a cheap alternative? what exactly is co2? can anybody give me the general information on substrates for planted tanks, and some secrets they use to maintain them?