on kensfish cascade heaters are like 11$ for the 300 watt. i have 1 on my 120 and it works like a champ and the 200 watt on my 75 and 40b. will be getting another one for the 65$ craigslist 135 im setting up 
yesterday I was doing a wc and other maintenance which caused me to to get out of my routine. I was just about to refill the tank and saw smoke at first I thought it was water vapor coming off the lense of my lights then I noticed smoke in the tank. Turns out I forgot to unplug my 500w titanium heater and it melted the heater guard in half. Didn't shatter but since it was supposed to shut off when out of water im going to make them replace it since its under warranty.
External inline heaters like the Hydor is the way to go, just connect it to the canister hose and your good to go.
i checked them out and they are pretty pricey and you would need a few for the 125 gallon