A couple months ago the nitrate level in the tank droped from the hundreds down to 20, and has stayed stable ever since.  I added red worms to each of the three beds maybe a month before that event, and had also added maybe a teaspoon of the seaweed extract every few days.  I also add a teaspoon of chelated iron every other week.

When the nitrate levels dropped, I figured sufficient nutrients had finally found their way into solution to allow the nitrogen to drop.  The tomatos just took off, and I was getting perfect 8 oz tomatos, basil, eggplant, and some really good romaine lettuce.

I have three grow beds with a total of 22 sq ft serving 25-28 tilapia weighing well over a pound each.  The grow beds are packed with plants and resemble cubic jungles of dense vegitation.  I say this to give an idea of the nutrient uptake capacity.

Now, ammonia level is flat at zero, nigrate = 20, pH=6.8.  All these values have held constant for the last 2 months.

Question:  When I feed the fish, they take what they need leaving their waste product which breaks down in such a way that the plants can grow well.  Now, the plants take what they need and probably leave unused nutrients behind that can I believe can build up over time.  Some of these nutrients can become toxic at higher concentrations.

Do folks have the experience of micronutrients build up?  What keeps this from happening?  If they do build up, what is the best way to handle the situation?

Thanks

Steve