Loose definitions are too blame. What is "clean" water? A good mechanical filter will remove a leaf that has fallen off a plant, but won't alter any disolved chemicals. A good biological filter will process ammonia -> nitrite -> nitrate, but will be clogged and killed by fallen leaves.
Aquarium filters should do both. A mechanical stage that removes floating crud and a biological stage to process disolved waste. The mechanical stage comes first, so the biological stage does not get clogged. It is importnant to keep the mechanical stage clean though otherwise the flow rate through the filter drops.
Carbon and Zeolites are rarely any use in a filter. They are inactivated very quickly. Unless you have a special requirement, (can't think of a regular one), you are better off with better media.
Aquarium filters should do both. A mechanical stage that removes floating crud and a biological stage to process disolved waste. The mechanical stage comes first, so the biological stage does not get clogged. It is importnant to keep the mechanical stage clean though otherwise the flow rate through the filter drops.
Carbon and Zeolites are rarely any use in a filter. They are inactivated very quickly. Unless you have a special requirement, (can't think of a regular one), you are better off with better media.
