Your fingers. Just gently grab the leaf where the bug is and rub a bit. Very soft bodies so easy to kill and I've got rid of tehm a few times over the years just by hand. I just pull the finger of the leaf thru my fingers and hopefully get any underneath as well.
Get yourself a magnifying glass like the big one on top of the pic. Can scan dozens of leaves in a minute and it has a tiny higher powered one built in for a closer look but I've found it to be the best for bug hunting. It's only about 3X in the big lens and 5x in the little one but works great and about 10-$15 at the local drug store. I use that little 10X loupe to check out my trichs to see if it's getting close to harvest and can take a closer look with that old Radio Shack 30x scope that also has an 8X magnifying glass on the other end that pulls out. Also have a 60-1000X USB scope but it's pretty useless to look for bugs without a tripod or stand to hold it steady.
These are the guys you go chemical warfare on when you find them on your plants. The Borg! I just use Safer's soap with 10ml/L canola or neem oil added for mites, thrips etc.
Seeing as how you have a chiller get the nute temps down to 65-68F or around 18C. I've done a lot of DWC grows since '01 and that's the sweet spot.