Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Some plants might be able to use CO2 better than Excel.
The constant (24/7) supply of CO2 is probably better than the 'boom then bust' supply of Excel as it is dosed then gets used up.
Anyway, I do not know how this relates to the death of the pleco, if at all.
Low NO3 is not a cause for worry. Just monitor it. As the plants grow better (improving supply of CO2) they will likely start using all the fertilizers in greater amounts. Since you say you have macros on order, I would think the tank will be OK until they arrive.
Temporary solution: Feed a bit more fish food.