12-09-2007, 10:09 AM
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well, if your heater is too small it will struggle to keep the volume of water in your tank at the correct temp. the temp will fluctuate more, and the heater will be on aLOT shortening its lifespan some, but the biggest issue will be the temp fluctuations. if the thermometer is too big, it will be constantly clicking on and off, and again you will have some monster fluctuations in the temp.
when I buy heaters I buy whats rated for my tank and round up. for example, for my 10 gallon tank a 10 gal heater is fine. for my 30 gallon tank I think its actually a 50 gallon heater. but that too, is a bit subjective. you may see a 100 watt heater rated for upto a 30 gallon tank (im just making up numbers here) but the next 100 watt heater you pick up is rated for up to a 55 gallon tank. so, keep that in mind when youre shopping :) if youre in the ball park youll be fine.