I am in the same position. Right now I am just venting some every few hours manually when I can in flower.
But I plan to get the hydro farm kit or a cheaper kit on eBay. It depends on the timer. If I use a 15 minute timer I co2 spew at .5 cubic feet every two hours to keep ppm at 1500. If I can a one minute timer I can vent at 10 cfh every two hours for 1 minute to keep at 1500.
I prob cant find a one mintue timer to work every two hours.
So I will buy the hydrofarm kit for 120 on eBay with all the fixings and vent 15 minutes with my standard timer every two hours. .5 will work fine.
Here is how I figured it out.
http://www.hydroponics.net/learn/ you might have to look aroudn fro their co2 calculator. I cant get the hang of copying the page as a link sometimes.
I got my tank on craigs list for a song and it was filled. It was not in the farm and garden area where they are three times what I payed and empty. look around.
I can already see a diference in plant growth and water and nute uptake. I have also switcherd to a t5 for veg. I hope I can get one to work for flower.
http://www.hydroponics.net/learn/co2_calculator.asp (here it is)
Co2 is easy.