That's a great idea. if you had 6 tanks and just 2 treatments you could do a t-test. Just do a 6+6 siesta vs a 12 hour photoperiod. For the sake of the experiment I would start with no co2, you could run a second experiment afterwards with co2 in all the tanks, or maybe excel. If you have a small scale that would help.
For response variables-You could measure plant weight, or plant height. Riccia is a good one for plant weight, or some java moss. Have a single crypt of the same species in each tank, weigh or measure them before and after. Also a stem plant would be good to include. Something easy and fast growing to get clear results. You could also count leaves on the crypts or swords. Fish would be much more difficult. If the tanks were large enough, guppy colonies would be hard to beat. Their population size after a few months would be a good indicator, as would their weight if you had a fine scale to weigh them. Algae would also be very hard to quantify. You might be able to quantify the % coverage on the glass or something like that, but I think it would end up being more like a subjective thing. Maybe for BBA you could count the number of threads on all your plant leaves? That would be pretty solid.