90 degrees is to hot and until you get that into the 70's range everything else you do is pointless.
Because it's so hot in your room your plants absorbed most of that solution to stay alive so I don't think it was from evaporation.
I found out years ago that every grow room that I built for clients I had to add an air-conditioner to control heat. Something most grow stores don't ever mention when they sell you all that expensive hydro equipment.
As for your
nutrient strength and evaporation question I will have to check my books and get back to you.
It is always a good idea to flush your system periodically to remove fertilizing salts.