Hi permies. Some questions for you that may be of general interest (take it as a school case study):
I have a small, plastic-lined pond (1.5 cubic meters – 1500 litres of water circa) with 20 some goldfish. The pond has a fair amount of aquatic weed (kind of Elodea, introduced by me), a couple of papyrus pots and a lot of algae (the water is green). There is no filtration system and I usually refill it with water only when it really needs it (no rain - high evaporation).
I am using pond water to feed my tomatoes but I have some doubts:
Which nutrients are expected to be in this water? Which NPK proportion? Minerals? Other nutrients?
Is it reasonable to think that this pond water should be used only with certain crops (heavy feeders? Fruiting vegetable crops?) and not with others?
To my understanding fish “manure” diluted in water is not composted, so using it should be like using fresh uncomposted manure. Is it? Should it be considered safe or not?
What about possible pathogens both for plants and humans?
Since algae is growing in your pond, it would be reasonable to conclude that there is a high enough input stream of nitrogen and phosphorus somewhere in the system. If the fish eat the algae, then the water should be absorbing some of the phosphorus and nitrogen from the decomposition of the fishes' feces and the algae that have died. I do not know what would be considered reasonable, but a good way of answering your questions is to conduct an experiment. It would be interesting to see the results. In the AP Environmental Science class that I took, we used Peaceful Valley Farm Supply's soil testing kit to test for the levels of NPK and the pH of the soil. Since water is a method of leaching, this could provide a clue. I have not tried the soil testing kit on just water, it might work... maybe? You could set out a few plants in pots, dilute the water to various, water the plants over a period of time, and see which plants do the best.
If your pond is not producing a stench, then aerobic decomposition is going on in your pond and you should not worry too much about it being "raw" fish manure. If your pond smells, then you have anaerobic decomposition going on. The same thing is still accomplished, but of course, a tad smelly and be a little concerned about the health of the pond. Decomposition in all its forms is necessary in ecosystems, but as a general rule of thumb, anaerobic decomposition is not usually safe for humans. So be careful.
Look at it with an ecosystem perspective:
-there is a pond
-there is soil around the pond
-it rains and water leaches away the nutrients
-the pond retains water
-the nutrients must be somewhere
-the algae are present (so we got N and P down)
-What goes on in the land you own?
-What do the neighbors do?
-Anything in the news about things you should be concerned about? Oil spill? Coal ash?
-Any industrial farms nearby?
-Any parking lots or roads nearby? Cars leak a lot, and what ever is in the car could be in the hydrologic cycle for your ecosystem.
-Do you have an underground sewage tank? Any leaks?
-Any pipelines underground?
-Where does the rain come from?
-What is the history of your property?
I'm not trying to worry you too much, but I am considering either Biochemistry or Biophysics as my B.S. when I go to college (one year of high school left). Those are just some questions that come straight to my mind for you to consider. If you are very curious about what is going on, you could contact a local college and ask them nicely if they will analyze your soil and water for you. They might even do it for free. Then, if you are very very curious, I would suggest you visit the DIY-Bio website. It is a website dedicated to the biohacker community: people who are interested in do-it-yourself biology and making science a more common and accessible subject for the general public. http://diybio.org/ Also, Ellen Jorgensen has a great TED talk explaining what the biohacker community is:
Forgive me for having digressed so much. I hope your plants do well.