a- yes you can totally suffocate your fish with Co2, you would be exchanging the oxygen for co2. The key is you need a day night regulator on it, at night it will build up unless you turn it down because the plants dont make use of it.
It is no joke, it will poison everything if you dont do a good chunk of research on it.
b- as far as costs, the mini cartridges imho are not worth it. even the lower end ones will end up sticking it to you with the cost in 6mo or less. What size is the tank? anything over 5gal is going to need something more robust to be cost effective to the kit that you linked. that kit could be okay on some nano 1gal or something, and then it might last you but still....
I would do diy co2 in bottles with yeast before I did the mini cartridges (I do now on my 10gal and i love it!)
the worry a lot of people have with Co2 really comes down to just knowing what your co2 output is going to be (your bubbles per min) You can also get a drop checker to make sure you dont dose everything in the tank and suffocate it. A drop checker is about 10-20 mins behind on the reading though and should be placed about midway away from the co2 diffuser.
Try a DIY set up imo, its easy you can test it, and track your growth rate with what youre working with! plus it is under 10$ to set up. I made it fancy and use mason jars for added decor!
Last edited by ao; 05-20-2013 at 01:59 PM.