Warning: Use Flourish Excel and CO2 Modestly!

One of the most unfortunate mistakes that planted tank beginners can make is trying to inject too much CO2 / carbon dioxide into planted tanks in order to stimulate plant growth. While this may be fine without livestock and may only cause some undesirable but harmless side effects, it can be quite deadly when you have fish, invertebrates or other live animals in the aquarium. Remember that live animals require oxygen to breathe. They expel CO2 into the water or air and it is a waste product. Injecting too much CO2 into the water, which is so easy to do with Flourish Excel, can cause your fish or invertebrates to suffocate due to oxygen starvation and carbon dioxide overload.

If you see that your fish or invertebrates have become sluggish, pale or seem lifeless, you will probably want to do a partial water change in order to dilute the amount of Flourish Excel that you may have added into the water. You may also see the fish trying to reach the tank's surface in an effort to breathe oxygen.

The excess of CO2 is not limited to Flourish Excel. If you're using any DIY CO2 unit you'll also want to monitor the water for CO2 concentration. Too much is too much for your livestock and the effects can be harmful and can cause death. So be prudent when using CO2 and less is best, adding more only gradually.