Hi,
I have had the Fluval M 150W heater in my 125l tank for about 7 or 8 months. I never had reason to inspect it closely, but I took it out today to clean the algae off, and noticed a small patch, about 1/2 inch x 1 inch, of condensation inside the heater, behind the glass. The silver paint inside appears to be gone where the internal water is. Luckily, I bought a spare Fluval Elite 150W heater and have put that in for now. My question is, is it okay to put it back in and use it, or is it unsafe? I much prefer it to the spare for looks, but I didn't want to chance using it before asking on here. I don't know how long the condensation has been there.
Any help is much appreciated!

If it's under warranty, contact Fluval; if not, bin it immediately. Either way, don't use it.