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Old 10-22-2011, 12:43 PM   #1 
Sena Hansler
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Question about heaters

I get asked about it a lot by people I know, and now I wonder too... Could you use a heater meant for bigger tanks in smaller tanks? I mean I have a 250 watt heater (meant for 90 gallons and up) but biggest tank is 29 gallons - but I won't use it because I'm not even sure it'd work - might just fry my fish ><
So on the behalf of myself and people I know in my area, how important is it to use a heater made for a particular size of tank? And why?

(mine is an adjustable heater, 250 watt Ebo-Jäger now made by Eheim, for saltwater and freshwater fish. So useless to me until I get a bigger tank xD)
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Old 10-22-2011, 02:35 PM   #2 
Oldfishlady
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With larger tanks (over 20gal) usually the higher watt heater will not cause any problems, however, you still would need to monitor closely and adjust as needed especially if the room temp is not stable.....but using high watt heaters in smaller tanks can be risky...even the adjustable type can vary in temp based on water volume and room temp...not as much as the pre-set type heaters but they will vary some...

You could use the 250w in the 29gal-but I would set the temp 10 degrees lower than you wanted-depending on the room temp...if its a really hot room you may want to go lower and a really cold room 5-10degrees and just monitor closely with a secondary thermometer-remember the larger the tank the more varied the temp can be in different locations/levels in the tank-especially under large items in the tank, plants and low flow filter.....
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Old 10-22-2011, 09:43 PM   #3 
Sena Hansler
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Hmmm interesting :) just didn't want to fry my fishies! Lol. Even my friend who I think has a 30 gallon heater has a 10 gallon.
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Old 10-22-2011, 10:13 PM   #4 
Thunderloon
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I won't get into all the sciency stuff of "specific heat" but generally if you look at the tank heating ranges of heaters such as the Hydor Theo series:

25w 5-10g
50w 10-16g
100w 18-30g
150w 30-50g

You see a pattern of overlap there. So you can expect a 25w to hold a 5g with a little overshoot but maybe not support a 10g in a cold environment.

If you assume that the smaller tank volume is what it will support in heavy air conditioning and the larger tank volume is what it will support with a full hood, good lighting and not much evaporation.

When they put a lower volume limit on the heaters they're figuring on standard water circulation for that size of tank as well as water quality and lighting conditions. So when we deal with Betta and reduced circulation or no circulation we have to place the heater directly in the water flow by the intake or output (or in some cases in the filter) to ensure that it doesn't produce a localized hot-spot that the fish can unknowingly swim in and out of.

The problem with under-temping a heater is that the more watts you put into a smaller volume, the larger the cold-to-warm swing must be to absorb the energy of the heater without over-temping and you get to a point where the heater ends up simply being an emergency anti ice age measure with the control setting so low that you'd have to spill a cooler in the tank to trip it on.

If you look at the range of the 150w heater 30g-50g its talking mid-range at 40g volume. Since statistics are used to determine the safety margin you can simply do the three sigmas on each side, 3.3g spread. So a 150w heater is likely going to be fine in a 25g tank with ornamentation but will suffer from over-heating the water in a 20g. I would assume it would be useless in 20g or less volume under "normal" conditions. If you happen to live up north and like to air condition at 70°F all the time and wear sweaters to save on your heating bill, you could probably get away with the 150w in a 20g but its still not safe.

Large surface area will make larger heaters "safer" for the fish, but its still best not to use the bigger heater in such cool environment conditions and go with two smaller heaters with staggered temp control instead.

Do remember that when they say "30g" they're talking about 26g, you've got rocks and often won't keep the water level at the top.
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Old 10-22-2011, 10:16 PM   #5 
Sena Hansler
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Hmmm interesting :) was wondering about that. Like... It'd fit the 29, but I'd have to keep it low (winter is colder mind you) and I do not wan to come home to find dead or sick bettas because of what you said - hot and come spots and such. The 29 will have a sponge filter, and has a light that's all I know so far.
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