Probably your bulb is burning brighter, but as they say, the brightest bulb burns out the fastest... (or something like that) Maybe the difference betwen 175 and 250 isn't enough to blow out the bulb immediately, but I would expect your bulb to fail at some point. Actually, I'm hardly an expert at this; you could post something on reefs.org for Sanjay Joshi. That guy knows everything about lighting.