Thankyou all for you comments
I clean them monthly, no problems. But they do cause a gradual decrease in the water flow. It also seems that the better the tank is doing the faster it builds up. It is particularly noticable in my spray bar (Same clear green plastic), presumably because it is under the lights of the tank and the build up slows my water flow.
from what fishy fanatic has said I'm pretty sure opaque tubing will relieve this problem (But not solve it). Will give it a go.