Temps dipped last night and we did not put the heat on yet. The fish tank room got chilly. The heater in the tank kept the tank at a steady 77-78 degrees. I was thinking if that heater ever konked out, do people use
2 heaters, 1 as back-up ?

All the time. I have a 100 Watt heater in my tank right now, and a 50 Watt still in the box as an emergency spare. Plus I have a 50 Watt on my 10 gallon and a 10 Watt on my 1.5 gallon.

I know many people that actually have both heaters running in the tank not only for redundancy, but so they can get a more evenly distributed temperature gradient throughout the tank.

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Officially fishless until after the move....tune in later for details!

If you meant to run both in your tank at the same time, I personally think it is the safest method. But, they both have to be heaters that are not capable of overheating your tank if one malfunctioned. Heaters can malfunction and never turn off and if you have gotten a bigger wattage heater you can kill all of your fish. If this happens with a low wattage heater, chances are it would give you time to notice before any damage occurred.

So, if you figure that you need a 300W heater (temp of room has to be known), you would get 2-150W heaters and put them at each end of your tank. Even heat and protected against a malfunctioning heater. I like to use controllers, which is another way to protect, but they will only control one heater.

I have an extra 200W heater but that's used for water changes to get the temp up for dumping the water back in. But if need be could always be a spare for one of my 2 tanks. Always better to have more than you need and never use it, then need it and not have it.

I was wondering whether to have 2 in the tank at once. I have a 29 gal tank. The heater that I have in the tank now is a 100w. (It came with
the aquarium kit.)
Is 5 watts of heater per gallon the general rule ?
Should I put a 50w in the tank also ?
Thanks for all of the help !

My room temperature routinely drops to 64 or lower (I'm cheap and I like it cool anyways) so I have 2 200watts in my 55 gal tank. Before adding the second one I'd see water temps dip to 72 when the room was in the low 60's, with the second one it stays near 78 even when the room is cold.

eb

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55 Gal
6 Buenos Aires Tetras
8 Black Skirt Tetra
4 Otocinclus
1 Dojo Loach

You can have as many heaters in your tank as you want LOL. I just cant see having more than one plugged in at any given time, unless your temps in the house/room drop to something crazy during the night to where one heater can't keep up.

The room temp fluctuates b/c there are 2 skylites and many windows.
(It was once a screened-in porch made into a sun room.)
No A/C in the summer and seperate heating system from the rest
of the house. I generally don't heat that room right away
b/c I can close it off from the rest of the house.
Now that there is a fish tank in there, it can get chilly (low 60's) in the room
during these fall nights with temps going down into the high 30's.

I have the same problem kind of. My 2 car garage has been converted into a bar and that's where I have my 40G breeder and my 10G nano. I too have 2 seperate base heaters in there so that room can really drop over night. Even with it insulated as well as it is. I have a 100w digital heater on both tanks and so far they can handle the swings. The 40G is set to 77 and drops to 75 at night. The Nano is set to 78 and gets down to 76. So that's not too bad but we'll see what winter brings.

How many watts per gallon just depends on the difference between what you want to keep your room at and what you need to keep your tank at. Last year I kept my thermostat at 63 during the winter and all of my tanks hovered around 78-80 and one tank staying around 81. The bigger the difference, the more wattage you need. There are charts to use out there on the internet.

Here is one of those charts:
Aquarium Heater Size Guide

Looks like I was right about needed 2 200 watts for my setup :D
eb

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55 Gal
6 Buenos Aires Tetras
8 Black Skirt Tetra
4 Otocinclus
1 Dojo Loach

Thank you all for excellent excellent advice ! We have a frost alert for
tonight, so I really appreciate it ! I think that I'll get an additional
50W heater, just to have on-hand for now.
I also need to bite the bullet (and/or oil bill) and crank the heat in the room.
I can then see how the existing heater is doing and adjust accordingly.
Here's to keeping warm !

Sounds like a plan, which is better than no plan. Glad we could help.

I have a heater rated for 20-30 gallons and have another rated 5-15 gallons in the same tank. It actually works really well. They kick in at night when the temps lower and I say they both run for a total of... 4-5 hours a day.

I would encourage you to pay attention to how much your heater runs this Winter. With the temp I kept my house at last year my heaters were on, seemingly, most of the time. So, knowing how crucial an item it is and given they are not terribly expensive, I have replaced most of them and will be doing more in the next couple of weeks. Not sayng that when next Winter rolls around in Dec 2012 you have to go out and buy a fresh heater, just saying be aware of how much that thing has been cycled on/off. Heaters do fail. Personally, I don't like how hard they get used for a few months and then they sit not powered for several months.

Great point, Ben, about how much "on & off" that heater is going to be doing during the next 6-7 months ! Even more reason to buy up a couple of
extra heaters. Who knows how good these heaters, that come in the
aquarium kits, really are ? Any brands that stand out ?