According to Coralife's website
, the instructions (page 5) state the following...
- 55 GPH to eliminate parasites, 121 GPH to eliminate algae, 253 to eliminate bacteria
- 110 GPH to eliminate parasites, 240 GPH to eliminate algae, 500 to eliminate bacteria
- 290 GPH to eliminate parasites, 680 GPH to eliminate algae, 1550 to eliminate bacteria
Even if the max GPH on the XP2 was between 200-250 GPH, the 18 watt would be sufficient enough to use to eliminate algae. Using a 9 watt, you would practically have to reduce the flow to almost half or less than the normal output, which lessens the effectiveness of the filter. You might as well just buy a small pump if you went with a 9 watt. Unfortunately, if you wanted to use the UV to eliminate parasites, even the 18 watt would not be sufficient, therefore, the 36 watt would best suit the needs. But as most of us use the UV to eliminate GW, then the 18 watt would best suit you. Although, if an event of a parasite outbreak, you may consider using a small pump to best utilize the UV.
With all this talk about GPH, I might look into buying one of the GPH flow meters
, since I have an Eheim Pro II 2128, FilStar XP3, and Eheim Ecco 2236. Unfortunately, the flow meter only comes in a 3/4" fitting and my filters use either 5/8" or 1/2" tubing, so the flow meter may not be as accurate. There is a more accurate flow meter here
, which allows me to use one for each tubing size, but at $100.00 (USD), it's not very practical at my income.