Unless you really made a mistake on the nute's, leave it be. If you gave them too much a dose, then yeah you could flush them, even then I'd do a light flush. Start by watching the tips to see if they brown, if they start to (overfeeding) you could flush. I'd really just observe them, unless them tell you there's a problem you'll likely cause yourself a problem overthinking the situation, and rationing extreme measures fucking you in the end, most strains are pretty resilient. If yellowing of course lack of N or too much causing a lockout, and if lockout, then flush, and I'd use a flushing agent, I like florakleen at full dose.
Fungus gnats? Must be roots organic soil... Hate that shit. I've never added anything to soil for my knowledge just doesn't know ratios and npk vaules to understand making my own soil. I just by ocean forest and leave it be. BUT I only have 3 soil plants, my mom's rest hydro, therefore buying ocean forest is practical, guys who run 24+ plants for efficency would probably attempt to make they're own soil. In that case I'd read sub's super soil recipe. my 2 cents good luck bro