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Note â€" If you donâ€™t want to read through all of this, just skip down to the second last paragraph.

I have seen posts where people want to know the cost of running their tanks for a month. Lighting is easy because the wattage is listed on the lamps and we can estimate the hours per day that the lamps are on. Filters are on continually so it is just a matter of checking the specs for the filter wattage.

The trickier part is the heater because it goes off and on. I tried watching my heater in my 45 gallon tank while I worked at the computer. I tried to record the time it went off and on for two hours. Sometimes Iâ€™d miss it though and would have to estimate. I came up with ON 22% of the time.

I have 5 tanks so I needed a better idea. Over two weeks I randomly walked by the tanks and checked off on a piece of paper if the heater was on or off. I tried to do it randomly, not every 10 minutes in case a heater was running say, an 8 min. off, 2 min. on cycle. I did this about a 100 times over many days but it took less time than sitting watching a tank for two hours. The 45 gallon tank was on 20.3% of the time so I feel my method was fairly accurate. The chart below shows my results.

180 gal. â€" 250 W heater = 1.4 W/gal. On 31.4% Cost per month is $5.21

90 gal. â€" 250 W heater = 2.8 W/gal. On 35.6% Cost per month is $5.91

45 gal. â€" 200 W heater = 4.4 W/gal. On 20.3% Cost per month is $2.70

33 gal. â€" 150 W heater = 4.5 W/gal. On 36.4% Cost per month is $3.63

23 gal. â€" 50 W heater = 2.2 W/gal. On 100% Cost per month is $3.32

All heaters were set for 77-78 F and room temperature in all rooms was 70-71 F so we are looking at 7 degrees heating (about 4 degrees C) above room temperature. All heaters but one were Ebo-Jagers (the other a Visi-Therm). All were installed in the upright position.

What amazed me was how efficient large tanks are at conserving heat. The watts/gallon figures are not used in calculating dollar costs but make for interesting comparisons. You would think that the more watts per gallon you have, the less the heater would be on. Surprisingly, that is not always the case. I was worried about heating a 180 gallon tank with a 250 W heater (would it be enough heat output?) but it is on less often than the same size and brand heater on my 90 gallon. I surmise that this is due to the larger volume of water retaining heat better and the fact that the glass is thicker. Also my 180 gallon is my only tank with foam insulation underneath so maybe that makes a difference. Other factors that could affect your heating are: the location of the tank (by a window, wall, open door), the type water movement in the tank, location of heat vents in the home, and the type and amount of covering for the tank. [So tell the spouse you need a bigger tank and it wonâ€™t cost any more to run it!!! (At least the electricity part)].

In case you wonder how to calculate the cost of the heater, I will give you an example. My 200 W heater is on 20.3% of the time so that is 20.3% out of 24 hours so .203 x 24 x 30.4 days in a month = 148.1 hours a month. 148.1 x 200 Watts = 29,620. I am paying 9.1 cents per kilowatt hour so that is 29,620/1000 x .091 = $2.70 per month. You can calculate your lights and filters the same way. I am calculating in Canadian dollars. At the moment (April 2008) it is on par with the U.S. dollar. I am not going to do international exchange rates so if you are in Europe or Australia; you are on your own.

I donâ€™t have live plants so I have single bulb lighting and the lights are only on an average of 6 hours per day (so for me I multiply watts of lighting by .25).

Examples of filter energy use are: AquaClear 110 â€" 14 Watts, AquaClear 70 â€" 6 W, AquaClear 50 â€" 6 W, and Eheim 2217 â€" 20 W. Amongst my five tanks I have 9 filters.

I calculated that the cost total for electricity for the 5 tanks I have, to be $29.91 per month, which averages out to $6 a month per tank. 70% of that electrical cost is for heaters, 22% for filters, and 8% for lights. If you live in an area where your home is above 70 degrees F much of the time because of the warm weather, your heating costs would be less. If you are heavily into lighting, that portion will be more.

Other costs would be costs of the water (I am on a flat rate at the moment, not a meter), and the cost of heating the water as you do water changes â€" most people would not be adding it cold from the tap but trying for tank temperature around tank temperature. That calculation is for another day.

So there you have it. If you are too lazy to do all this math and recordkeeping, you can use the $6 a month per tank average for light, filter, and heater expenses if your electricity rate is around 9 cents per kilowatt hour. A 12 cent/kwh rate would be $8 a month per tank; 18 cents works out to $12 a month.

I have been back into fish keeping for the past five years. I have put off doing these calculations because I really did not want to know the answers. The costs were not as expensive as I had thought. You might think Iâ€™m crazy to go to all the trouble math-wise, but I must confess to being a math teacher. Say, I think I might use this as a bonus question on next weekâ€™s quiz!!