I won't go as far to say that it would never freeze because if it gets cold enough, it will still freeze but that would require much lower temperatures (e.g. < 0F).
A 60 watt (incandescent) should provide more than enough heat.
If interested in the physics of freezing, you can read this thread.
As for the other questions:
http://www.poolforum.com/pf2/showthr...x-and-Freezing
http://www.poolforum.com/pf2/showthr...c-acid-storage
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