FYI -- Live fishes in the water do not cause it to smell fishy. Otherwise it would ALWAYS smell fishy since there is always relatively the same amount of fish biomass in the pond throughout the year.

Fishy smells coming from the water are caused almost always by certain algae blooms. Certain species of algae produce metabolic by products that "stink" or cause tastes/flavors. The odors can smell like various things and fishy is one of them. Also some of the by products of these algae give water offensive tastes or odors. I won't go into naming the offensive metabolites. But the metabolites are fairly aromatic/volatile and readily dissolve into and evaporate from the water.

The exception to this is when you have a lot of dead fish present and the decompostional odors can produce a fish smell to the water. But live fish do not make the water smell fishy; live fish are basically clean and odor free. The odor or nose test of fresh fish in the market is an example of what I'm talking about. Cleaned fish that smells fishy is starting to spoil. Bacterial spoilage of fish flesh does not take long once the fish dies and is exposed to the air and warmer temps.

Another example is fish such as catfish at catfish farms that occasionally have an off flavor. Catfish farms always taste test their fish before harvest. This off flavor is caused by algae blooms in the pond and the fish absorb the algae metabolites. Reduce the algae and or put the fish in clean water and the off flavor dissipates fairly quickly.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management