As others mentioned here - your problem is likely stability rather than the carbon dosing itself.
Big fluctuations as well as bottoming out nutrients can indeed cause RTN/STN to occur.
The trick is to know how to manage them correctly and balance the system by daily tests and carful micro adjustments until stability is achieved and daily dosage (if needed) is determined, then a weekly test will be enough keep track of your nutrients.
Personally I’ve been using a permanent carbon source in the form of Biopellets - many reefers who tried to use this method wrecked their systems, but I’ve been using it successfully for years, reason being is that I have plenty of phosphates to spare and I’ve determined a daily dosage of nitrate to balance it back, which in turn takes a part in my phosphate export strategy in the system.
Point I’m trying to make here, is that no matter what method you choose to use, wether it being a carbon source or a fuge - it needs to be used in a controlled manner.