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.
Thanks, hadn't thought about the statistical analysis but a t-test makes sense. What do you mean by a 6+6 siesta, though? 6 on, 6 off, 6 on, 6 off?
I would keep the plant species to one so that interactions among plants aren't an issue (for example, one taking up more nutrients than the other).
I like the idea of guppies/population size for measuring the fish, but that could get tricky if you happen to throw in, say, 3 females that are on the verge of having babies in one tank versus 3 that just had babies. Instead of total population size I guess you could instead count how many fry are born, how many survive, etc., although from experience it's kinda hard counting all those little things!
What about fish behavior? What would be a less-subjective way to describe that? I'm curious to know if the split photo period stresses the fish, so I'm wondering if there are specific behaviors to observe.