interestingly when i had to put the 'T' into the 40mm downpipe when adding the skimmer last week, I had a look inside the pipe. there was a very light coating of brown algae - about what you'd get inside your skimmer neck after an hour! (in other words, bugger all) other than it not being worthwhile, it would be very difficult! transparent piping is a little different as the light can cause the algae to grow.