Duck weed

[Photoman]

I read that Duck Weed is good for the aquarium. Provided you keep it in check.It multiples fast. I have some in all my aquariums. The weed is growing roots. It does look good on top of the water. My tanks are planted and has floating plants already. Does Duck Weed keep the nutrients in the water stable? Thanks

[Diana]

Any fast growing plant will be removing most of the nutrients it needs from the water.
Plants with their leaves in the air (including floating on the surface, like Duckweed) will get their CO2 from the air, so are not limited by the low CO2 levels in the water. This means they can grow as fast as the nutrients in the water permit.

Nutrients in the water come from several sources. If those sources are regular in re-supplying the nutrients that have been removed, then the nutrients in the water will be pretty stable.

Fish food is high in nitrogen (N), phosphorus (P) and most trace minerals (about half a dozen elements that plants use in very small amounts)
If you are feeding fish or shrimp in the tank, monitor the NO3. You can use this as a guide to how much phosphorus and most traces there are in the water.
If the NO3 is rising, then the plants are not using the nitrogen, phosphorus or traces as fast as you are adding it, so you need to do water changes to keep the NO3 levels reasonable. This also keeps the P and traces low.
Fish food is low in potassium (K), iron (Fe), calcium (Ca), magnesium (Mg) and sulfur (S). If the plants are not even using up the N and P, then it is possible that the lack of these other nutrients could be slowing the plant growth, especially K.
Low light levels can also cause this, or just having a small amount of plants in the tank.

Water changes with a GH of at least 3 German degrees of hardness is likely to have enough Ca and Mg, for the plants, though some water is exceptionally high or low in one or the other. Plants use these in a ratio of about 4 parts Ca: 1 part Mg. But some water has very skewed ratios. If you suspect a problem with GH you could find out more information from the water company, or from water tests, so you know if you need to dose one or the other. Or you could simply dose a GH booster that has both Ca and Mg. (Careful: some GH boosters include sodium chloride, salt, that a fresh water tank does not need). Generally, if the GH suits the fish, then it is probably OK for many plants.

Lack of sulfur is not usually a problem in fresh water tanks. Even if it is low in fish food, usually there is enough. Also, if you are dosing fertilizers, or any of several other things they may have some sulfur. Dechlor, for example has some.

This leaves K and Fe as the two elements that are usually the first to be lacking in most planted tanks.
If you need to dose these you could start with something simple like Leaf Zone, a liquid that has both K and Fe. If you want to use separate products to fine tune the dosing, then use a chelated iron.

Plants that stay underwater need to get their carbon from the water. Roughly half the plants we grow can utilize carbonates as a source of carbon. The others need to get it in the form of CO2. This is in low levels in the aquarium. The natural rate at which CO2 enters the water is pretty slow, and the under water plants can remove it all in just a few hours. The amount from fish is not very much.
There are liquid carbon supplements like Seachem Flourish Excel and similar products.

To address the other part of the question:
What is considered 'stable'?
A parameter that rises and falls in a regular cycle (such as daily or weekly) and stays within the acceptable range for the flora and fauna in the tank is stable.
Examples:
N, P etc rise through the week as you feed the fish, then you do a water change. So the levels stay between 10-20ppm, and the fish and plants are happy.
CO2 rises through the night when plants are not using it, then drops through the day as the plants remove it. Then, overnight, the levels recover. This is stable.
K and Fe get used up as the plants grow, but you add more every other day, at about the rate the plants use them, or just a bit more, then once a week do a water change that resets these at a lower level. This is stable.