hi gchoo
you will only need one AC fan for a 4footer and the last time I left it on for 24hrs the temp reminded a constant 27C to 28C...which was alittle worrying as it is a discus tank....so what i did was to connect the fan to the same timer switch as the lights so that it comes on and off in line with the lights...power consumption is quite marginal, and the brand I bought was the Ista brand.
I dont think the fish is going to get sick if you can hold the temp swing in the 1C range.
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