I use Excel every day/every other day as directed as the only carbon source in my 12 gallon tank. It works very well and i can definitely tell the difference when I use it - there's no new algae growth and the dwarf sag foreground starts growing fast and growing much taller.
I have also used it in my big tank to kill BBA by squirting the excel directly onto the algae with the filter off. Kills the algae dead in a matter of days. I don't have good luck killing algae by double dosing the entire tank and I'm fearful of the extra Excel harming my catfish and fry, which is why I use the squirt method. I also will occasional put a partial dosage in the big tank to boost the CO2 levels if the diffusion isn't great.