Depends on a few things. Cost, available space, what sort of pump you will be plumbing it in to and whether you want clarification or sterilisation. On the cost front it seems that good uv's aren't cheap and cheap uvs aren't good. And if you want high level sterilisation then you need lower flow rates, however if you are limited to using the current pump you have on the tank and let's say its a fairly high output pump then you won't be able to get low flow rates, unless you tee off a separate line for the steriliser, or get a large output steriliser. For cost effective sterilisation I'd go with a good brand pond UV. Something in the 50+ watt range. You can pick one up for around £100. The TMC 110w is a beast of a UV unit. And only cost £180.
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