Heating a 125g

[nes999]

i feel werid now i use 10 watts per gallon and on my 55 i have a 150 and a 300. on my 500 gallon i have a 850 watt pond heater with 2 300 watt heaters

 

[bdog]

granted, I could probably run the tank off of one 250w heater, but in winter it would be on most of the time and if it ever failed and I was on holiday, I could come down stairs to a cold tank of dead fish..so I use two..

This is why I thought people ran more than one heater, redundancy. That's my excuse anyway

 

[mdb_talon]

This is why I thought people ran more than one heater, redundancy. That's my excuse anyway

I always run two heaters. If one malfunctions and gets stuck "on" then chances are it will not fry my tank before I notice. If it fails and shuts off completely the other one is going to likely be enough to keep it "warm enough" in the time it takes me to notice the temp has dropped a few degrees.

To the OP I would go with 2x200.

As for people "overdoing" it that is real hard to say without knowing the environment the tank is in. I have open top tanks where at night I let room temperature get in the low to mid 60's. Some of the tanks are close enough to the windows that when it is below zero outside the actual air temperature at the tanks can even get to upper 50's. It is rare, but you need to size your heaters to handle those "rare" occasions also. On my 120g reef I actually had to switch to 2x300w heaters because at night the water temp was dropping to between 76 and 78 but during the day it was 82 to 83. Not a huge deal for most freshwater fish, but very bad situation for a reef tank to have a 4-6 degree fluctuation in temps.