Fish primarily get their nutrients from their food, which is then provided to the plants through waste; however some vital nutrients, like iron, calcium, or potassium, may not be found in fish food. There are six nutrients, referred to as macronutrients, that are necessary in large amounts for ideal plant growth. These are nitrogen, phosphorous, potassium, calcium, magnesium, and sulfur. Additionally, there are other nutrients that are necessary in smaller amounts; iron, manganese, boron, and zinc.
Iron is necessary for plants to produce chlorophyll, which is partly what helps give plant's leaves their green color. Yellowing leaves, therefore, could be a sign of iron deficiency. This can be solved by adding lead-free iron supplements, like chelated iron. However, this will only be effective if your water's pH is 7.5 or below. Calcium is used by the plants in order to transition from blossom, or flower, to fruit. Plants who aren't getting enough caclium can show signs of necrosis, or blackening/death of young plant tissue. Calcium deficiency can be solved by supplementing calcium in the form of hydrated lime (although this will also raise the water's pH levels). Low levels of potassium are usually diagnosed by spots or scorching in the more mature parts of your plants. You can directly spray potassium supplements onto your plant's leaves, or by adding potassium supplements to your fish's food.
