Ermm.... fish poop.
I keep aquariums, and one of them is a goldfish tank. Very big VERY dirty fish, so I scrape lots of black sludge off the filter pads into a sprayer bottle. Sometimes I add a little chicken poop, and sometimes I add Miracle Gro. Everybody seems to like the fish poop.
Before I had my lazy man's hooks-up-in-the-sink siphon, I used to use the drained water from water changes to water plants. It really seemed to help them thrive.