hi ob i normaly change my rez once i have added the same amount of water as is already in ie 20ltr rez say your plants drink 3 litres a day i would change after 7 days as ive added 21 litres of water.as for rez temps i keep mine at 64f to 66f any higher than 70f and you run the risk of root rot.hope this helps lol ph1l1p72