I've never used Excel before, only DIY CO2 and starting on pressurised CO2 shortly. With most fertilisers it's common practise to wait a month before dosing, but unlike Flourish, Flourish Excel is NOT a fertiliser, it's a carbon supplement to take the place of CO2 in the aquarium. I've never heard of having to wait before using CO2, I'd think that Excel is the same. Best to listen to the experts though, perhaps someone who has used it before can comment?