No material for growing plants is available in as many brand names as fertilizer. The two primary types are solid fertilizer, which is mixed in with the substrate, and liquid fertilizer, which is added periodically to the water.
The first kind is useful when planting deep-rooted plants, but they do have a major disadvantage. If algae is becoming a problem and over dose of fertilization is the suspected cause, it is impossible to dig up the bottom layer and remove it. Therefore, it is better to err on the side of caution when adding base sand fertilizer. However, substrate fertilizer has a powerful effect on plants that can be clearly seen, since it is absorbed by the roots.
Liquid fertilizer, however, is absorbed by the leaves, and is therefore not going to have a dramatic effect on the plants' condition. On the other hand, the amount administered is much easier to control by simply changing the water. Most aquarists use liquid fertilizers, but I do prefer the effectiveness of substrate fertilizer.
One way to avoid problems associated with using solid fertilizer is to closely monitor the aquarium water for cloudiness when initially filling the tank. Then change the water repeatedly until it clears up.The fertilizer will eventually settle into the sand and stop floating up into the water.
Almost everyone uses liquid fertilizers but almost no one uses them well. The instructions rarely take into account the amount or type of plant. Fish foods rarely tell how much to add to such and such a size aquarium, because it is easy to figure out how much by watching the fishes. If they haven't eaten everything, no one would add more. However, plants aren't as easy to figure out. Unfortunately, when plants are looking bad, a lot of people seem to assume that they need more nutrition and add more fertilizer. Most of the time, this just winds up killing the plants
Plant health can be determined by noting the pH value, whether the leaves are shiny, if new buds are appearing, if algae is growing on the plants, and other such simple observations. The amount of fertilizer shouldn't be changed until the various aspects I have discussed are all checked out. One exception is whitening of leaves and buds. This is a clear sign of malnutrition.