I was able to turn the mogul socket sideways, so that the bulb will point directly down at the tank.. It was quite a nostalgic moment, thinking the last time this screw on the back was loosened was 25 years ago. There is not much clearance between the socket and where the glass begins, but the light I've ordered is only 6 cm long including the base. I'm getting a LED screw in bulb with 3x 3w diodes in cool white. Advertised to yield 600 lumens so it should get me some very bright lighting. Not much of a gamble for $4.50. Worst case, if this thing is too long, I can take the led bulb apart and wire it up directly-bypassing the mogul base.