Chemical filter
Uses adsorbent materials to remove dissolved substances from water that mechanical and biological filtration cannot eliminate.
Detailed explanation
Chemical filtration works at the molecular level. Adsorbent materials capture molecules dissolved in water by binding them to their surface.
Activated carbon is the best known chemical filter. Removes yellowish colours, odors, drug residues and many organic substances. In freshwater aquariums it is often used after treatment with medicines to remove residues. In marine aquariums it should be used with caution because it can also remove trace elements useful for corals.
Antiphosphate resins (iron or aluminium based) are essential in marine aquariums to keep phosphates low. They are placed in the flow path in a reactor or in a bag in the sump. They must be replaced when their adsorption capacity runs out.
Ion exchange resins can remove specific ions from water. There are resins for silicates, for nitrates, for phosphates. Some are regenerable, others disposable.
Zeolite is mainly used in freshwater aquariums to remove ammonia. In marine aquariums, zeolite systems (ZEOvit) represent a complete method of ultra-low-nutrient aquarium management.
Purigen by Seachem is a very popular synthetic resin that removes organic compounds and makes water crystal clear. It is easily regenerated with diluted bleach.