A few weeks ago the leaves of the plants slowly started dying. They first turned yellow, eventually you could crumble them in your hands, but the veins were still visibly green. The new leaves also turned yellow. In the end the cucumber plant couldn't produce fruit, or they would end up completely yellow and sour tasting.
A lot of tests and research were done. It had to be some kind of deficiency problem, but we weren't actually sure which nutrient was the real cause. After a lot of research it became clear that we had an iron deficiency. The fish poop on which the plants grow doesn't contain any iron. Usually the water in the system contains iron, but after a longer period of time the plants will use it all and then the problem of the iron deficiency starts.
We ordered some seaweed powder to solve the problem. After adding the seaweed powder we did some iron tests again, but still no iron was to be traced in the water. After more research, adding chelated iron could solve the problem.
So, we searched the internet and found Ferrrilene: a red chelated iron with a 6% of iron in it. The only problem with red chelated iron is that is turns the water in the systems deep red. Because we weren't sure if the red water would be bad for the fish more research was needed and the company where the Ferrilene had been bought was contacted to make sure the fishes would cope with the water turning red. At the end of the day iron was added to one system to monitor the process.
Almost immediately after adding the iron, the water of the system turned deep red, but the fish seemed to be OK. So iron was added to the rest of the systems.
Now two weeks later, the plants ,especially the cucumber (who also had the most trouble dealing with the iron deficiency), are doing much better. All the new leaves have a beautiful, green colour again and there are a lot of fresh cucumbers. A week after adding the iron the cucumber got harshly pruned and a lot of dead, yellow leaves and bad cucumbers were
cut off, leaving the plant with some bare parts. There were also parts of the plant where the leaves recovered themselves and the new leaves also turned nicely green again.
After adding the chelated iron, the iron level in several systems rose significantly: it went from completely 0 to 1 mg/l. This 1 mg/l is the iron level we want in our systems, so the chelated iron appears to do the job.