Garden ponds are more than just water. They are biologically active habitats for flora and fauna. Ponds are subject to constant change through the seasons. However unlike natural bodies of water, garden fish ponds are limited in their capability to regulate their biological balance on their own.
The primary cause of illness in fish is due to poor water quality, which is caused by either an overstocked pond or an undersized pond filter.
Most pond filter unless stated otherwise do not include a pump, pumps usually need to be purchased separately, exceptions include the Oase Biopress and the Filtral filters.
German made Oase pond filters are unmatched in quality. They are more expensive than many others on the market, but the superior materials and professional design means these filters will last much longer than their cheaper cousins. The warranty periods on Oase are what you would expect from a high quality product. Oase pond filters are designed with ease of cleaning in mind and many come with built in UV clarifiers for killing algae and bacteria.