Relax Brah. Your tank temp is OK.
As long as the water is clean and the light is excluded you can run as high as 90 F. in the res.
Maybe higher, ah dunno.
But I've run near 90 for 3 weeks and did not change out the water at all during bloom.
I use no preventative other than what's already in my tap water and have yet to see any problems, (3rd grow).
Course, that could just be luck.
H.S. is right about the PH.
You'll want to keep hydro at 5.2 - 5.8.
Don't have to be too ridgid about it, they benefit from a little PH "drift".
I see you are using Hydrotron clay pellets.
I'd advise you to start your res water at 5.2 because the H'tron will try to raise your PH constantly, even when properly conditioned.
You can use that fact to schedule PH adjustments and sweep the range where plants use all the basics well.
Shake is also right about your form filling.
It's a pain, but it's why you have not been "being served":D
Aloha,
Weezard