Ammonium or nitrate uptake?

>I have measureable ammonium in my tank. Can somebody explain to me why I
>need to add nitrates to avoid nitrate deficiency for the plants? The nitrate
>is used up by the plants but I thought that plants prefer ammonium?

You're right, if you have measureable amounts of ammonium _or_ more than
trace amounts of nitrate, your tank is not nitrogen limited, and adding
nitrate will do no good.  The fact, however, that you _have_ measureable
amounts of ammonia(um) is a sign of trouble.  In a fully cycled tank with a
healthy biological filter, even without plants, there should be no
measureable ammonia(um) or nitrite.  In a "fish" tank, you will see rising
nitrate levels until water changes reduce it.  In a planted tank with
strong growth, the plants often use _all_ the available nitrogen, and you
never see the nitrate level rise either.

Karen Randall
Aquatic Gardeners Association