Rusty bulb sockets. Ooops!
Background: I documented retrofitting an Eclipse Six with the Hellolights.com 28W kit here: http://www.plantedtank.net/forums/ta...t-co2-56k.html
Now, five months later, the bulb stopped working. I removed the bulb and plug from the fixture and noticed that the bulb was loose. A little more wiggling caused it to fall out of the socket!
I sealed the plug with silicone, but not the bulb. The bulb probably gets too hot for the silicone, but when the next one arrives I'll try it.