Thanks for all your suggestions! In response:
-the tubing is already completely buried, except for what's underwater.
- the tube from the pump to the waterfall is about 15' long, and yes, quite
windy, bendy.
- I have no ideas what the gunk is. The water flow is not slow - it's a
1000gal pump. I have no water problems, it is clear.
-I don't think Grubber's idea to blow will work, the stuff is really
attached to the walls of the tubing.
- thanks Craig, for the company. Such quick drop in flow rate might mean
you need a filter on your pump, that it's your pump that is getting clogged,
not necessarily the tube. Maybe some things you read here will help you.

I've always had this problem, regardless of other varying circumstances.
Through smoth and ribbed tube, clean and dirty water, with or without UV.
I think I'll try the electrical fishing wire method next, and get some of
the sharp bends evened out a bit to make the job easier.
Thanks everyone. I'll keep watch for more good ideas.
Anda in S. Ontario