03-07-2013, 12:36 PM
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Heaters can malfunction either way--overheat or fail to heat. I have had both problems, and sadly lost fish as a result.
As for thermometers, the glass mercury tube thermometers that float in the water (you can attach them to the glass with one of those little suction holders) are the more accurate. I bought some of those digital strip thermometers that you stick on the outside back in the 1990's and I have noticed that they are not accurate. Though having said that, they do seem to be constant, just not at the actual temperature. The glass tube thermometers are worth having.
On the heaters, my Tronic have been operating flawless since 1996/7. I bought a couple of the Fluval digital ones, one failed within a year and the other is questionable. I am now buying Eheim Jager heaters to replace as needed.
A couple of points to keep in mind. First, don't scrimp on heaters [not implying you did, this is just "general" advice for everyone reading]; buy the best you can, as no piece of equipment is so important. Filters can fail, but this will not result in fish losses before it is noticed and resolved; but a failed heater can kill a tank of fish overnight.
Second, buy larger wattages; these heat faster, so they are working less.
Third, have more than one in any tank over 3 feet in length, and have them next to the filter intake or return so water is moving past them constantly.
Fourth, make sure the ambient room temperature is not more than 10 degrees below the intended tank temperature; heaters are not intended to heat "cold" water of in cold spaces.
Fifth, for tanks with canister filters, consider one with a built-in heating element. I have one Eheim Pro II with this, and it has never failed in continuous oeration since 1997 and the tank temperature never fluctuates more than 2 or 3 decimal points from the set temperature--i.e. set at 25C and it varies from 24.8 to 25.2, and that is solid reliability.