brown spots might mean burning
the light might be too close
certain plants dont need Co2
and a good light is fine
such as anacharis (common river plant) and ferns
other plants, the pretty leafy ones and little grasses most people choose really dont do well without CO2
. I am just adding a C02 system myself.
since you have a 15G, you dont need any paintball tanks so make your own like the other person suggested, but I have a prebuilt canister called hagen
, you can find it on amazon for like 20$.
also, look into moss. they are pretty and easy kept, usually.