It comes with a second, less powerfull empeller, it puts out 1600 gph instead of 2100. Here are some specs of the mod: Sure Flow 1600 Kit: Premium Aquatics _________ 55 gallon slowly growing reef aquarium, 85 lbs. live rock, 2 1/2" sandbed, 1 False Percula Clown, 1 Sixline Wrasse, 1 Mandarin Goby, Tetratec PF500 filter, 2 110 watt URI Super Actinic VHO actinics, 2 250 watt metal halides with Reef Optix 2 reflectors powered by Blue Wave 3 ballast, born 1/3/08 My Tank Thread My Tank Video  |