I used to notice similar gasping at the surface in the morning in my tank when using Excel. I even went to the point of staying up all night and testing O2 levels every hour over the course the evening with a LaMotte O2 kit. I found that O2 dropped from a starting level of slightly over 7 ppm (near 100% saturation at my tank temp) to close to 2.5 ppm by morning. This was not a CO2 issue at all.

My theory (and this is obviously only conjecture) is that the use of Excel can cause significant O2 depletion because it drives plant growth in a heavily planted tank to the point where, during lights-out, O2 is taken up so heavily by the plants that its concentration in the water column drops to dangerously low levels for fauna. There may also be some other oxidizing chemical reaction(s) driven by the polycycloglutaracetal in Excel that causes the depletion. In any case, as was stated earlier by Carissa, aerating the tank at night will alleviate this problem. Stopping aeration an hour or two before lights on and kicking in the CO2 supplementation should prevent any major swings in CO2 levels when photosynthesis starts.

Excel kills bacteria and other organic materials, these are typically reduced compounds that are both biologically and chemically oxidized when O2 is present.
So O2 will be removed while this occurs, about 8-12 hours.........fine during the day, but not something I'd do at night or at high levels.

I've seen folks kill fish.
It can be very species selective too.

Regards,
Tom Barr