I don't know what levels it should be at, but the main thing that will
be affected is your fish not getting enough oxygen. If they look like
they need more o2, increase surface agitation by adding a bubbler or
something. Be more worried at night, when more co2 accumulates. I hear
some people run a bubbler at night to help it out a bit.

Dave M. Picklyk wrote:
Hey guys, I'm ecstatic with my DIY co2 setup. Right now I'm getting about
26ppm (up 7ppm from yesterday which was a day after I started). What are
considered harmful levels? If there are more plants in the tank, I'm
guessing logically that more C02 will be used up thus decreasing the ppm is
this right? I'm going to be getting a ton of plants hopefully in the next
few days so this might be a good thing to help use this C02. (I think it's
kinda being wasted on a few javas and an anubias nana )

If my c02 keeps pumping up to toxic levels how can I stop it without cutting
off the supply and guessing when to introduce it again?

Thanx for all you help!
Dave.