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Old 11-25-10, 11:16 AM   #13
Piwoslaw
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The fish don't need to swim in the phase change material, they can swim next to it.

Find a material with melting point around 80°, maybe some kind of wax. Cut it up into really small pieces, or thin slices (lots of surface area), wrap those in plastic (so it doesn't get messy) and throw it into the tank. When you start pumping hot water into the tank, the temperature will rise until the wax's melting point. You keep pumping hot water, but the temperature in the tank doesn't budge until everything has melted, then it starts to rise again. The temperature curve will flatten out again when cooling off until the wax has become solid again.

So, if you know the melting point of your wax, then you can keep the temperature variations just a few degrees above and below that, so the fishies won't care.
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