In any case, I wrote a quick note to Don Bailey, to see what he thought of the high nitrate levels. Don and the rest of the UVI team are professional scientists, growing tens of thousands of pounds of fish and vegetables annually, using aquaponics, year in and year out, so I value their opinions, to say the least. After he scolded me for starving the fish (sorry Don!), he said nitrate levels are safe up to 500 mg/liter. That's the same as 500 ppm (parts per million). He also said I definitely have a secondary source of ammonia, given that the fish had not eaten for an entire month and that nitrate was still present.
The only obvious source for that ammonia which later gets converted to nitrate, is fish waste and other waste (plant roots, leaves, and other material) trapped in the gravel grow beds. It is likely that the red composting worms are responsible for turning that waste into ammonia, which then gets converted by bacteria into nitrate. The ammonia measurements I took of red composting worm leach ate, which came in very high, support that thesis.
As a result of this interaction, we started feeding the fish again last Tuesday. We started them on a very light diet (1/4 of what they normally got for 2 days, and then 1/2 as much as what they were getting previously), as I was afraid of giving them too much after so much time without food. Also, because of the secondary source of ammonia, I want to make sure I do not add so much food that nitrate levels shoot up beyond the 500ppm level. As a matter of fact, I may leave the feed level at around this rate, or even reduce it, depending on nitrate levels.
Again, my goal is not to grow tons of fish. I am more interested in plant production. Therefore, my goal is to maintain nitrate levels that will result in the maximum amount of plant growth, without maxing out on fish feed and fish growth. I believe that if there is a way to accomplish this, it is by saving fish waste and letting it decompose over time in the gravel grow beds, assisted by red composting worms, rather than by removing it from the system (as is done at UVI and most other places).
In my measurements today, nitrate clocked in at between 100-200ppm. The particular kit I have maxes out at 160ppm, so I took a second reading of a 1:10 dilution for more accuracy, and came up with the 100-200ppm reading that way.
Here's the video for the week. Besides the water chemistry discussed above, there is also an update on the plants, including cucumbers, tomatos, and lettuce.
PS: I harvested 6 fully-grown lettuce heads today! Next week I should have a near raft-full (18) of lettuce ready to take out. It's getting to be time to upgrade the system. I'd like to be cranking out 100-200 lettuce heads a week, and have room for other stuff.