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Since the water will equilibrate, I would pull and return it to the sump. Use a temp probe in the display and adjust the chiller as needed.

My main reason, other than keeping all that I can in the sump is that a localized cold area could do some harm.....

in the sump, it has a chance to mix with wth other water and move back into the main display.

As far as the pump.....I may be the minority, but I prefer to buy the cheaper via aquas as a workhorse on these auxillary circuits and relplace them more often than buying something like a mag.....which from what I heard produces alot of heat, but reliable. I just prefer to replace the cheaper ones every year or so to minimize failure.