Most tank plants won't benefit from CO2 because of poor light or more likely poor nutrients in substrate.
I'm sure most here know that plants breath oxygen, they eat CO2.
Even very low light tanks will grow plants better if you use CO2. Tropica research has demonstrated that. And, those of us who have tried it have also noted it.
Also adding CO2 can increase or decrease pH depending on carbonates.
Adding CO2 will always decrease the pH. That actual pH depends on the KH, but the change in pH with added CO2 is always a reduction.
Most importantly, increasing CO2 can kill fish in marginal O2 tanks because of partial pressure issues at the gills. Excel is also a reducing agent so it can also kill fish in marginal O2 tanks.
Excel, used per the Seachem directions, has not been shown to harm the fish in any way. All aquariums need a good dissolved oxygen content in the water. That is the oxygen the fish need to live. With or without CO2 being added, the fish need oxygen. With high levels of CO2, the fish can live with the CO2 much better, and at higher levels, if the water is always near saturation with dissolved oxygen.