All tank inhabitants, from fish to plants to invertebrates to bacteria, can be killed by too much iron. Iron is a heavy metal (as is copper, zinc, manganese, and some others) and thus toxic, but iron (like the others I mentioned) is also a necessary mineral for fish and plants. Most water conditioners detoxify heavy metals; this in itself should tell us that there is a danger. The thing is to ensure sufficient iron is present for what the plants need, but no more.
Fish obtain their mineral requirements from fish food and water. Soft water fish species do not need minerals in the water, but hard water fish species do need the "hard" minerals calcium and magnesium. Iron in the water column is not going to benefit any fish or invertebrates.
Several decades ago the idea emerged in the hobby that iron was the "missing" ingredient for aquarium plants. But this can be taken too far, as iron is a micro-nutrient, and in excess it does more harm than good to plants (and obviously to the fish and invertebrates). An excess of iron can cause plants to shut down assimilation of other nutrients. Iron should always be minimal, and in proportion with the other 16 necessary plant nutrients. A "comprehensive" fertilizer like the one essjay mentioned is the safest way to ensure this; the Brightwell Aquatics product which is basically identical to Seachem's Flourish Comprehensive Supplement for the Planted Aquarium is called MultiFlorin.
Concerning the API Leaf Zone, I would not use this. It is only potassium and iron. Some claim to have improved plant growth using this product, and it may well be that iron and potassium are missing in their water. Fish foods that pass through the fish will provide the nutrients plants need as they are broken down. Sometimes this is all that is needed, depending upon the fish load, plant species and number of plants. When this is not sufficient, and "fertilizer" is deemed necessary, it is safer to use one of the comprehensive supplements mentioned above. Both of these products not only have all essential nutrients, but they are also in the right proportion to each other, and that is important to prevent an excess.
None of these products are "good" for fish or invertebrates, so using them minimally is always wise. And substrate tabs, at least the Seachem Flourish Tabs, are safer for fish, invertebrates and plants because they only release nutrients into the substrate water and these are taken up by the plants rooted in the substrate. Floating plants and non-substrate rooted plants obviously will not benefit from substrate tabs and a comprehensive liquid is (or may be) required.
