Thanks 4 reply AndrewHope u are wellI have 20 inch pre-filtersand yes a little sunlight does get through.Is there anything I can use to stop this happening?apart from the obvious-moving it inside.How about wrapping pre-filters with black bin liners or something similar.Thought I paid about £40 last time for pre-filters so don't reallywant to change 6 weekly if I can avoid it.Could this all be damaging the membrane?Russ