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Watts is what you want to look at. The GPH or flow rate is just the rate you need to have the water pass through the light to get maximum kill rate.
Shop around because some 25 watt lights are rated for a 125 and others are not.
I had a aqua ultraviolet with the wiper cleaner thingy... I do not like the wiper thingy because it always leaked after I used it. The rest of the light was good.
I would recomend the coralife turbo twist 18 or 36 watt UV. The 18 watt needs 300-400 GPH and can handle up to 250 and the 36 nwatt needs 600-9000 GPH and can handle upto a 500 gallon tank. Those are the manufacturers ratings and i think they should actually be half of what they are as far as tank sizee is concerned.