With diy co2 I doubt that you will ever produce enough co2 to kill your fish. But either way if theres too much co2 in your water you will notice your fish hovering at the very top of your tank (closer to oxygen) breathing rapidly like thier gasping for air.
Oh and your cannister can not regulate the amount of co2 in the water. With diy co2 the only thing regulating how much is in your water would be you yourself. The best way to achieve this would be to mix the proper amounts of yeast and sugar in the bottle. In other words less yeast = less co2. But again I doubt you will have too much co2.