Any idea what's causing the edges of my lettuce leaves to brown?

I'm a total noob and built my first hydroponic setup out of PVC and a bucket (and my 3D printer). I started this a lot later in the year than I had hoped, but I figure I'll finish it off in the basement with some UV lights if it gets later in the year.

I'm in Western Canada and the temperatures have been quite warm in the past week. About 36-37C (mid to high 90F?). I have a small fountain pump that keeps everything pretty well moist at all times. Reading would lead me to believe they're not getting enough water, but as you can see the pods are saturated. The other thing I've noticed is the leaves are a bit yellow.

I am doing water changes every 2 weeks and fill the bucket once or twice a week. Every 2 weeks during the water change I have been adding a 2 part fertilizer to the bucket in a low dosage called Quick Grow 1-2-4 GROW (part 1) and Quick Grow 2-0-0 PRO NITRO (part 2).

The only other thing I am thinking is maybe it needs to be out of the sun for a bit longer in the day as it's getting cooked. I could be totally wrong, but the fact the plants are smaller than they should be at this time of year means maybe they're not as hearty as they would be in this heat if I had started sooner?

I'm open to learn and appreciate any feedback. Thanks!