ooh i tried layers, didnt work out for me. the best gravel for cheapest price is pond gravel, surprisingly high in nutrients and i filled my 55gallon with 1 bag, about $16. you do understand you dont need Co2 for a good planted tank, i dont use any and ive gotten great growth. of course i got the tank and the preasure gauge, PH monitor, and difuser, but honestly i cant figure it out so i dont use it (if any of you guys wanna help me figure this out i would love it btw). if you want the cheapest for a large tank, go to a river, but honestly, man made/collected cheapest items is playsand. go to home depot and buy a jiant 40lb bag for around 5 bucks. this stuff is great, just add some fert tabs or liquid fert every so often, and if you got the right light almost any plant will grow, ive gotten, baby tears, dwarf hairgrass, and much more hard plants to grow rapidly in my tanks.