Question
i have a 600 gallon pond,the water has gone black in colour and you can only see about 6in down,i have a blanket weed machine,uv filter and have used chrystals,the blanket weed is slimy.the water has been checked and that was ok.

Answer
Some algae may appear black.  Do you think it's algae or something else?  The UV only kills things that flow into it.  If the black is in the water, the UV should kill anything living in that.  If you have more attached hair algae (blanket weed) than anything, then the UV won't work on that.  I suggest manual removal and natural methods of balancing the pond such as adding plants, good filtration and aeration, good bacteria, barley straw, etc.  My page on algae is at http://www.fishpondinfo.com/plants/algae2.htm
What is in your pond?  I hope things improve!

Robyn