The same holds true for phosphate. Water changes help keep the nitrate levels low in the tank as long as the tap water itself is low in these substances. In some areas, the tap water can contain excessive amounts of nitrate due to agricultural run-off. In other places, municipalities are adding phosphate to tap water to decrease lead leaching from old pipes. In both of these instances, the aquarist may need to resort to a purification system of one sort or another to remove these substances from the water.
Most tropical plants require a great deal of iron.
Even if your water source has high concentrations of iron, it is unlikely that it will remain in a form usable by the plants for more than a few hours after it is added to the tank. Unless you have a very low tech tank with slow growing plants and low lighting, you will probably need to supplement with iron. I currently supply iron to my plants in several ways. I use laterite in the substrate of all tanks. I also use plant food tablets that are high in iron.