I don't know if you can do this with your tanks, but I have my 75 gallon elevated with 3/4 inch square rods and have ice packs under one end to create a gradient. My ambient temps are around 80F, which is too hot for me to turn on my heat lamps. The ice pack doesn't create a "freezing" zone, but it does cool the substrate. Zorro can choose where she wants to be.

Hope this helps!

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