The following article is about aquariums but
it has some hints that could help you pinpoint
the source of your problem.

http://www.thekrib.com/Plants/Algae/red-algae.html

kathy :-)www.blogfromthebog.com
this week ~ a snake story

Pond 101 page for new pond keepers ~
http://hometown.aol.com/ka30p/myhomepage/garden.html