Thanks for comments/info.
I've ordered the rot-stop.
Also ordered a couple of submersible water pumps.
The plan is to have a second bucket with 2L bottles filled with ice, a water-pump in each bucket to circulate the nute mix from DWC, into the chiller bucket then back.
The pumps have adjustable flow rates which will help in tuning the system. I've read that a 2L bottle of ice takes around four hours to melt at 30C.
Can't wait to get my Autumn hydro grow on and not have to worry about keeping the res cool.
Will have to mod my dehumidifier and incorporate the auto topup system for whenever i go away next.
Thanks again for advice,
steely