if you cant bring down the temp any,and 81 is not too bad anyway,I've found that using a silicon type additive will make them a little more capable of dealing w/heat stress and some other stuff too.I currently use a house brand from my local hydro shop.

well theres a pic,I was trying to put in a link but you can see the site name on the bottle.I use it as ph up when I'm mixin my nutes.good luck on the grow