Thank you @magnata!

Only those who do nothing do not make mistakes.

Indeed that is such a true statement!
The flower stacker contains Phosphorus (P) and Potassium (K), so it should be good for flowering. They recommend using after 3 weeks of flowering. That seems kind of long to wait, maybe I can try to wait 2 weeks lol.

It is very important to observe the doses of mineral fertilizers: neither an excess nor a lack of minerals does anything good with them!

I have to get some testing kits and also learn how to test and change PH when necessary too.
I still have a lot to learn.