Altum wrote:

Ammonium is just the protonated salt form of ammonia.

NH4+ -- NH3 + H+

Put it in alkaline water (as in most ponds) and it promptly loses a
proton and becomes ammonia.

Actually it becomes ammonium hydroxide. It may never become ammonia.
Ammonium hydroxide is alkaline by definition. The fact that the
ammonium is in salt form keeps the ammonium hydroxide from becoming very
strong. That is what chemists call buffering. If what you said was
true, all bottles of household ammonium hydroxide would explode since
ammonium hydroxide is alkaline water and ammonia is a gas.

Successful water lily food manufacturers use ammonium compounds:

The Once-A-Year Aquatic-Spikes from AgSafe (a div. of AgriTab Corp.,
Clearfield UT) contain:

Ureaform, Ammonium Phosphate, Potassium Nitrate, Potassium Sulfate,
Calcium Sulfate, Ferrous Sulfate, Iron Sucrate, Magnesium Sucrate,
Magnesium Sulfate, Manganese Sucrate, Manganese Sulfate, Zinc Sucrate,
and Zinc Sulfate in a time release format. The are safe for fish and
other aquatic life. The nitrogen in their formulation is 70% ammonium
based and 30% nitrate based.
--
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18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA