My guess is it would take about 2.5 hours for a 2000W heater to bring the
tank from 0 to 50C. For slower, use a smaller heater, and stir. You can
achieve 0C by putting ice in the aquarium until it stops melting. If you
want slightly less than 0C add some salt first. Would this be slow enough
? If the tank walls are insulated with polystyrene it could take two days
to go from 0 to ambient.

Peter

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