That's a really good point. I run one heater on my 75 and I've run into trouble because of that. Great suggestion, Becca!
Yeah, we're replacing the Eheim Jaegers on the 150 with 2 Cobalt NeoTherms or whatever they're called because we've got a lot of money invested in that tank. I wish I could afford to swap out all of my heaters for the Cobalts, honestly, they're a lot easier to work with and seem to be more on-target with the temperature I have them set at.
I love Eheim Jaegers, too, especially as an "economy" heater and I've been known to make use of "used" ones, but if you've got stock you really have a lot invested in, get new heaters, don't get used ones.
If someone is dead set on getting used ones, I can sell mine once the replacements come in, they work and the Jaegers are a quality brand, but it's not what I'd recommend.