"Nikki Casali" wrote in message
...

At a certain concentration CO2 becomes toxic to fish, I believe.

Depending on whose numbers you believe that is somewhere above
30ppm or 40ppm. According to Tom Barr, you should shoot for
20-30ppm of CO2. Apparently, there is little point in going above
30ppm because CO2 levels above that make little difference to
plant growth. I've gone to 30ppm in my tank with no ill effects
on the fish.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com