Help! What's happening to my Serrano plants?

Hi All,
 
It's my first time growing in deep water culture and my serrano plants have been doing quite well. I'm using an 11 galon home made DWC system with lots of aeration and using a 400w lamp in the evenings. Nutrients are Foxfarm Big Bloom and Grow Big Hydro with EC 1.8 and a ph around 6 or a little higher. The only issue I had run into before are high tank temperatures (high 27/28 degrees) but I can't do much about that because im in the tropics. There's lots of ventilation (the system is set up next to the entrance windows of our store) and the roots are nice and healthy.
 
Yesterday I notices some discoloration on the leaves of the smallest plant in the system. One of the other plants also has a small spot that looks like the start of the same thing. Can anyone help me and tell me what's happening? The solution is new, I just changed it yesterday since the previous week it was too high.
 
Thank you!
 
Tropicalhydro
 

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