what i've seen/experienced due to my lack of attentiveness, if you overdose with excel, you're doing the same thing if you inject too much co2 into your tank (especially if you're already injecting co2). your fish will start to gasp at the top of your tank, however ottos/plecos/etc, cannot gasp properly at the top of the tank. as a result they sufficate and die.I am glad you mentioned that. i was thinking of getting some. How are they sensitive? I mean what happens?