Fluoride is found naturally in freshwater at concentrations of 0.01 – 0.3mg/l, and in saltwater at 1.2 – 1.5mg/l. In some regions, fluoride is added to tap water at a concentration of around 1mg/l, to reduce tooth decay. Whilst this is not a problem for marine fish and invertebrates, it is higher than freshwater fish experience in their natural environment.
There has been relatively little research done on fluoride toxicity to ornamental fish, so much of what is known comes from work on fish that we do not keep in aquariums and ponds. The toxicity of fluoride to freshwater fish depends on water temperature and water hardness, being more toxic in warmer, softer water. For sensitive species in soft water (e.g. specialist Discus aquariums), the fluoride levels present in fluoridated tap water might be sufficient to affect behaviour and long-term health. However, it is unlikely that it could cause any immediate harm.
As water hardness is reasonably high in many areas that are fluoridated, and many ornamental species are quite hardy, it is unlikely that the levels included in tap water will cause any harm at all. Flouride added in tap water will be removed over time by plants and other biological / chemical processes.
In most cases, it is unlikely that any action needs to be taken. However, concerned aquarium and pond owners can simply ensure that they only replace a portion of water at a time. For example, a 25% water change, using tap water containing 1mg/ fluoride, will give a total concentration of just 0.25mg/l. The need for using tap water in aquariums and ponds can be further reduced by using Tetra EasyBalance and Tetra Pond WaterBalance.