As would be expected, plant growth does not appear affected by the diminishment in fish feed. That's because of the residual nitrate in the system (see Part I).
The exception is with the lettuce heads, which have not grown as quickly. The only explanations I can think of are:
These are different lettuce plants, genetically. I doubt that's the case, but it's a possibility. The lettuce starters come from the store and so I have no control or knowledge of whether or when they change seed lots. Still, lettuce seeds are bought or produced by the thousands - no, 100's of thousands by any starter-producer, so I highly doubt I happened to catch the "switchover" to a new batch or breed, and even if that were the case, I doubt one kind of lettuce is going to grow that much more slowly than another.
More likely, the difference has to do with some potassium I added to the system two weeks ago. Potassium or calcium should be added periodically because these are usually lacking in fish feed. Also, the nitrification process lowers the pH, slowly over time. Either potassium or calcium (in potassium hydroxide or calcium hydroxide forms) can be added the raise the pH, and since they are essential for plant growth and missing in fish feed, they are convenient to use because they will not kill the plants or build up over time. As a result of the potassium I added, the pH went from about 6.6 to about 7.2 or 7.4. That's much more of a swing than I expected or wanted at one time, so I'll add much less next time. By the way, this pH swing was caused by adding a mere 2 tablespoons of KOH to a 4,400 liter system!
But anyway, I am making a big deal out of relatively nothing. The lettuce heads did grow, just not as quickly as past batches. In the meantime, everything else is growing like gangbusters.
Tomato plants that were barely 7 inches tall 3 weeks ago are now 40 inches tall. Their stalks went from sphagetti thin to well over "pencil"-thick. Cucumber plants, in the system 2 weeks, easilly doubled, if not tripled, in size.
You can compare the size of everything in the video below (compared to last weeks video):
Fish starvation will continue until nitrate levels come down. Hopefully that won't be more than another week or so.
The interesting thing about all this is that it shows that aquaponics can be used on a much less intense fish-farming level than is widely assumed, or at least than was assumed by me! For those more interested in plant production and not so much in fish production, that is very important, moreso if one is trying to produce food commercially. Fish feed is one of the major input costs in commercial aquaponics. After electricity to run water and air pumps, it is the #1 cost in most places.
If fish waste solids can be recycled in the system rather thrown out, then the fish feed necessary to grow plants can be reduced, along with costs. Red composting worms play a key role in role in this. I doubt it could be possible otherwise.
Obviously, there are a whole lot of people that have spent a lot more time studying all this than I, but based on limited but real results, I say it's worth looking into some more. I've gotten here by mistake, and I have possibly endangered my fish by subjecting them to high and extended periods of nitrate levels. But the principal appears sound - namely that solid fish waste can be kept in a system that includes gravel-based beds innoculated with red composting worms, where it will be processed by the worms and reduce the amount of fish feed necessary to grow plants. Some balance between the amount of gravel-based beds (worm habitat volume), worm density, fish density, steady feed rates (unlike mine!) and plant growing area must exist. Isn't there an App for that?
Here's another video, this one more about the plants than water chemistry. Also some nice flowers at the end.