Water which has become discoloured and appears a brown-yellow or 'weak tea' colour is a result of organic tannins being released in the tank. This will happen over a long period and is simply the result of normal biological processes which break down waste products.

A more severe discolouration can be caused from tannins released by bog-wood, which again is quite normal but can be unsightly. Tannins are not harmful to your fish and in some cases can even be a health benefit.

Whilst it is often advised that soaking bog-wood for a number of days, or even weeks, can prevent tannins being released, this is rarely the case and the wood will still release tannins (although the effect will be less severe).

Using a filter media like activated carbon will remove tannins, and can be used on a permanent basis or as and when required.