Get a single stage unit Ranco Controller for about $80 on ebay IIRC. They are the same thing as the Aqua Logic Temperature controllers which are actually just re-branded Ranco units.
I run a dual stage (heater & chiller control) unit. Best money I've spent (well, aside from the chiller its hooked up to).
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32"x32"x20" Cube-ish tank
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