2nd indoor grow, dwc, switchable 400w mh hps

after countless hours of non stop reading on here(by the way thanx all you guys) ive decided to try a dwc system. my first indoor grow was with soil and cfl's and that didnt turn out like i wanted it to so im trying to do it a little bit bigger and better. i have a 3'11"x3'11" grow tent with a 400w switchable mh hps for vegging and flowering. i have a 36 gallon res with 6 3" pots. 4 of my seeds sprouted on day 3, im using fox farms trio at 1/4 strength and keeping the ph around 5.8. the temp in the tent stays around 74 degrees during the day and the humidity is around 50. my only problem is that the water in the res stays around 71, sometimes a lil higher, like 73. i kno its not safe to let the water get over 70 degrees. any ideas on how to lower the water temp? only 4 days into the grow so i will b posting pics weekly, would greatly appreciate any advice during my grow. thanx

five days into the grow...

day 6........

day 8. Put tape over pots so light couldn't get in the tank. No nutes added to ph 5.6-5.8 water yet.

Roots 8 days old.

Day 11
Changed tank and added more bubblestones and another air pump. Still no nutes, just ph adjusted water.

day 12
plants look fine except for one is drooping and has little blackesh spots on one of its first leafs and one of the others has little brown tips, any idea what it is? humidity stays between 55-59, temp 71-74, ph5.7-5.9 and the water temp stays around 69-72. added nutes today, less then 1/4 strength, ppm 161.