Ultra violet lights and biofilters 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 pressure systems (which I do NOT recommend) the UV light comes after the pond filter because the whole system is under pressure.
Whilst it is not applicable in many cases you can split the flow from your pond pump - part through UV light and part through the pond biofilter.

Use garden pond bottom waste on the compost heap. This organic matter is full of mainly anaerobic bacteria which will catalyse the composting process. Backflushings from the pond filter can be sprayed onto the lawn or garden as a source of nitrogen fertiliser.

Use black or opaque pipes in your pond instead of transparent low cost plastic tubing. Since light cannot penetrate the black piping the algae growth inside the pipe will be very considerably reduced.

 This means less maintenance and also less expenditure on replacing transparent pipe when it becomes clogged with algae.