Like FP, I find that the N tanks in ffof as they go into flower, and needs a substantial boost to make it to the end... they def. use A LOT of N through flowering.

I add dry ferts to the soil and fine tune things with liquid nutes when I water... I think if you don't you'd need to use more liquid nutes which generally cost more.

I have found gnats in most soil mixes from time to time, just use some nematodes and yellow sticky traps and they will be gone...