In the US, our typical household electricity is 120 volts, but it's the heater wattage that is the number of interest. The target wattage of the heater(s) depends on how cold the room is and how warm you need to make the tank water. First, take the average room temp and subtract that number from the desired tank temperature to get the total degree of temp increase desired. Basic rule of thumb for a 55"ish" gallon tank:
To raise the water temp above room temp by
* + 5F = 150 to 175 watts.
* + 10F = 200 to 225 watts.
* + 15F = 400 to 500 watts. (Requires 2 x 200W heaters )
Your wattage needs may vary a bit depending on how much water is in the tank, tank cover (or not), and room air flow around the tank.
NOTE: on any tank 4' and over, I recommend using two smaller heaters rather than a single large one to reach the total wattage. That way, you can buy two slightly higher total wattage-than-needed heaters, giving a reserve capacity, and even if one heater fails, you aren't likely to fry or freeze your fish before you notice a problem. Two heaters also gives much more even tank temps if you place them correctly.
In my 55 gallon, I have two 100 watt heaters and elevate my tank to about +5 degrees above the room temp.
My only addition to what DKRST has suggested is to have higher-wattage heaters. Two heaters in any tank over 3 feet in length, but higher wattage which tend to be more reliable, at least in my experience. I myself would use 200w or 250w heaters (two, one at each end next to the filter intake and return).
I have a needed gain of 17+ degrees in the winter so I changed my heater for an inline 300 watt unit. Overkill for a 37 gallon but I found my 150 was iffy and running more than I liked so just going with the 200watt didn't seem like enough. The thing I like about inline is that it gets everywhere as it heats the water before the spray bar.
I think that, if you are using a canister, it might be worth a look.
Byron has a good point about upping the wattage a bit. My house temp stays very constant so the lower wattage works well for me, but leaves me very little "extra" in reserve if one heater should fail during winter.
The in-lines are nice. I can't bring myself to spend that much on a single heater, there is always something else I seem to need, but they do have some advantages!
My in line (Hydor 300Watt) was only $60... I don't think that was much more than a decent non-glass submersible... I'm sure that the Aqueon Pro 150Watt was at least $40 or so. Twice the wattage for 50% more cost... I'm sure I could find cheaper if I looked hard enough but that's not a bad trade off. Besides, I sent the smaller heater into the office tank, I had to buy another heater anyway.:roll:
For what it's worth, I have two heaters (Aqueon Pro) in my 60g, a 200w and a 250w. I like the Aqeuon's because they are plastic coated aluminum (nearly indestructible) and have the red light / green light indicators so operation is clear at a glance.
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