Nymphaea lotus green?

yes once u get over 2.5wpg u really want to start using co2.(just about all planted tanks benefit from co2 is some form)

U can use excel but in the long run u might want to get a pressurized system for more efficient use and cost.

Using the directions on excel u dose between 7-8ml of excel for a 65gl. 1 500ml bottle will last bout 2mths and run about $12 a bottle. so a year of standard use would be bout $72. Assuming youre not over dosing.

If you're in the 3-4wpg range u might not want to short yourself on co2 or ferts

This is just my opinion. Im no where near an expert on this stuff but i coulda saved alot of money by not buying or buying the right stuff. Although I do hav alot of extra stuff around for "just incase" or if i ever want to start another tank

http://rexgrigg.com/ is a really good site to buy regulators and for general information on co2, ferts and light use.