How do you lower a high cod ppm result in a tank of water?

Usually the only way to lower a COD in water is to install an expensive biological system which must be maintained and in most places requires a class K operator. The best way to keep the cod low is to deal with it before it enters the tank of water, because treating it is expensive. Usually dry scraping things and throwing away refuse, rather than power washing things will help to lower the COD. Be careful of purchasing anything, for instance, people may try to sell you on a treatment system for metals which is not what you need, but as many people do not know about water treatment they end up purchasing something they do not need. For most purposes, an expensive biological system is required to treat high COD. Though, if you find a way to continuosly aerrate the tank with tiny bubbles, this should lower the COD but probably not by much.