How can a pond be cleaned?

we are looking at a house with a natural pond. its quite dirty. there is some debris that needs to be removed. however, the water is black. i would like to see the water to be a more natural color. there is alot of algae. there are frogs, but no fish.(when there is a drought, it dries up) but the water is sooo black. is there anything natural that we can do to the water to clean it up? i don't want to use chemicals, because the horses next door use it, and herrings eat the frogs. i know natural ponds are not chrystal clear, but its just nasty. is there something off balance in the water? something i should look for? a test to see if the water is ok? something that wont hurt the wildlife and horses that can clean it up? any info would be great. TY