 However your change seems strangely high to me from 0 to 0.0 let alone 0.00. I wonder how it can change so far? Do you know your tank water KH? AC Y yme Lifetime Charter Member Lifetime Member Nov 00, 0000 000 00 00 Mar 00, 0000 #0 sorry to say... but I have the same oxyguard CO0 analyzer as tom has and I have to say that I don't use tapwater in my tank --> 000% RO water with sera mineral salt. KH 0.0. pH 0.0 gives 00 mg/l. But I measure 00 mg/l. and the change goes fast with 0 rhinox diffusors. and as far as too much CO0, i don't know for sure but I gues that you have to go really really high before you see toxic effects. greets, 