My res temp is 26c - 79f

should i just keep adding frozen water bottles to help maintain a cool res

Bro I had res temps of 80 n I just used hydroguard. Seriously. But ugh yeah you'll need a chiller because your water pump will gradually generate heat over the course of time regardless of the size. Than you're subjected to lower DO rates in the water, root rot, and stressed roots. They never shipped hydroguard to you?

I've brewed 2-3 EWC teas for soil and never got the tea to foam like everyone claims its supposed to. /shrug.

make sure ur sterilizing the bottles themselves b4 u drop them in... very bacteria friendly environment in the root chamber...

My air pump sits where my fresh air intake pumps in (from outside) so my temps stay pretty low in the flower room.... veg room, not so much...

Done all the tricks to drop a degree or two? (block the light from the res w/ insulation or similar, fan blowing at the bucket, dont have 12ft of black air hose sitting in the light (done that) ......all the little shit can add up...)

This is a recent RDWC I made myself. It isn't flawless but it doesn't leak or clog so I'm still using it despite some design over-looks. Learned a lot though from putting one together myself..... Anyways heres the link(s) if your interested.

MY DIY/Self-taught RDWC grow;

http://rollitup.org/t/the-vumar-grow-rdwc.858399/

MY older DIY DWC from a few years ago;

http://rollitup.org/t/blue-widow-dinafem-dwc-3.508214/