This is a shorthand way of saying that one molecule of ammonia reacts with one molecule of water to form one ammonium ion and a hydroxyl ion. From the double-headed arrow, we can tell that the reaction can go either way and hydroxyl ions and ammonium ions could combine to form ammonia and water. This is precisely what happens as the pH of water increases, that is, the water becomes more alkaline. You may recall that alkalinity is caused by an increase in hydroxyl ions. An increase in hydroxyl ions (or alkalinity) pushes the equilibrium to the left and more un-ionized ammonia is formed.

At any given time, there will be both ammonia molecules and ammonium ions present. The quantity of each species is dependent on both pH and temperature.
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To Recapitulate: Ammonia exists in two forms in the water:

Ammonia levels in excess of the recommended limits may harm aquatic life. Ammonia toxicity is thought to be one of the main causes of unexplained losses in fish hatcheries. Although the ammonia molecule is a nutrient required for life, excess ammonia may accumulate in the organism and cause an alteration of metabolism or increases in body pH. Different species of fish can tolerate different levels of ammonia but, in any event, less is better. Rainbow trout fry can tolerate up to about 0.2 mg/L while hybrid striped bass can handle 1.2 mg/L.

Fish may suffer a loss of equilibrium, hyperexcitability, increased respiratory activity and oxygen uptake, and increased heart rate. At extreme ammonia levels, fish may experience convulsions, coma, and death. Experiments have shown that the lethal concentration for a variety of fish species ranges from 0.2 to 2.0 mg/L. Trout appear to be most susceptible of these fish and carp the least susceptible.

Elevated levels can also lead to ammonia poisoning by suppressing normal ammonia excretion from the gills. If fish are unable to excrete this metabolic waste product there is a rise in blood-ammonia levels resulting in damage to internal organs. The fish response to toxic ammonia levels would be lethargy, loss of appetite, laying on the pond bottom with clamped fins, or gasping at the water surface if the gills have been affected. Unfortunately, this response is similar to the response to poor water quality, parasite infestations, and other diseases.

Experiments have shown that exposure to un-ionized ammonia concentrations as low as 0.002 mg/L for six weeks causes hyperplasia of the gill lining in salmon fingerlings and may lead to bacterial gill disease.