help fellow growers

ok im being tortured by the brown root thing. i have an aerflo2 60 site setup going in a basement. the water in res has a fish tank heater in it set to 70 the room itself can get sorta cold after lights go off. that is the only thing that i think that could be causing this brown root thingy. my nutes are at 1260 4th week and flowering(autos...lowrider 2) i have had brown roots since week 2 and now they just look really shitty they are not slimey and temps dont get over 72 ever. plants look really good and will post pics if i even get a reply to my post. ph is very closely checked every day and is corrected. drain tubes are all the way down to allow the most o2 to the roots so i know they get enogh air. if there is anyone out there that can help i would greatly appriciate it. im pulling my hair out.

i have changed the res twice so far in 30 days, room gets no colder than 60 lights out. i use adv nutes 3 part and floralicious.