Fertilizers are food for plants. Unlike people or animals plants do not have a tendency to store away the excess energy from food in the form of fat, which is a condensed energy source. If there are excess nutrients in the soil or the water for aquatic plants, the plants simply do not use them. Most of these nutrients are in the form of ionic bonds or salts. If these are not used, the salts build up in the soil. This is part of the reason you end up with plants being burned; or in the case of aquatic plants, you get melting away of leaves and stems.