CichlidFish.com - Fish That Think.

what heater should be used for a 100 gallon aquarium?

IMO ebo jager 200 watt have always been my favorite for large tanks. Of course if they have a ebo 250 watt then that is the way to go.

Depends on at least 2 things = the size of the tank AND the ambient room temperature. The bigger the tank and/or the lower the room temp, the bigger the heater needs to be.

I have a Hydor Theo.
This one is the 400W model. I got this one because our 90g is in the recroom in the basement. It gets pretty cool during the winter. Ambient temp varies between 60-75F.

I could also have gone with a pair of 300 wattage heaters. Using dual heaters would mean even if one breaks down, there would still be one working.

I would suggest anyone with Oscars or other larger semi- aggressive fish to use a heater guard on in-tank heaters. Our 2 Oscars have knocked our heater off the back of the tank while chasing each other. It's kinda nerve rackin to hear that *TiNK*CLINK* and find your heater sitting amongst the rocks on the bottom. Firmly believing in the "ounce of prevention/pound of cure" type things, I put a guard on our 400W. Oscars have been known to deliberately attack heaters too. This could have something to do with the red LEDs used & what's humorously referred to as the "BullFighter" effect.

There are two other "safe" options I could mention:
1) A titanium heater.
2) An External heater. This requires some extra equipement for installation - tubes/hoses and a pump.