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Man has kept fish in ponds through much of history - firstly for food where ponds were farmed much like land.
The 14th centry image below shows a pond being emptied for food fish. It would have been essential for these early fishkeepers to understand how to try and keep the fish healthy or they would, no doubt, go hungry.
More recently we have turned to keeping fish for ornamental reasons, and the choices, patterns and colours have dramatically increased.
There are many different types ponds and types of fish that can thrive in them. Most modern ponds have advanced filtration that is based on mechanical, biological and sometimes chemical filtration. Without effective filtration, pollution such as NH3 can build up and poison the fish.