Question
I have a 5-6,000 gallon pond with plants and goldfish. There are two biofilters up top that feed into a pond that drops over three ledges into the main pond. We just purchased a UV light appropriate for the size pond we have. The ledges and surrounding rocks are thick and black. Not green like algae. The water is not as clear as it could be and there is "stuff" (brown fuzzy)floating on the surface. I know there is algae but what is that ugly black goo? The pH is ALWAYS 8.5-9.0 no matter how much pH down we use. We used something for pond sludge this spring which did not help. I am ready to fill it in!

Answer
Hi Lori,
Thanx for your question.  I'm wondering how long it has been since the pond was last cleaned.  An ornamental pond should be cleaned about once every three years.  I let the water level drop, remove the fish and plant and then bail the remaining water (or siphon).  It's usually pretty filthy at the bottom.  Thick, black, slick sludge is usually the by-product of rotted bio-matter (dead fish, plants).  Do you have enough scavengers in the pond?  Snails,  bottom feeding fish?  Floaters will help keep down the green and brown algae.  Water lettuce, azolla, water hyacinth (these are illegal in Florida and some other Southern States, refer to your local County Extension agent for substitutes).  A small bale of barley (available at most places that have a watergardening specialty will also keep down string and other forms of algae.  I know this probably isn't what you wanted to hear but it is what I would do if you're still experiencing black sludge.  It may be time to clean it out top to bottom.  I hope this helps.
Tom