Must the UV be placed before the biofilter?
In theory it does not matter at all but in practice if you have a simple box type filter is must be before the filter. Reasons are: The UV works under small pressure – water must be forced through it while the biofilter drains under gravity. In sand filter systems (which I do NOT recommend) the UV light comes after the pond filter because the whole system is under pressure. Not applicable in many cases but you can also split the flow from your pond pump – part through UV light and part through biofilter.
