Ion-exchange media like the one mentioned in this thread certainly has a place, as canister filter media for example. You want to place it in a stream of pressurized water, if its coarse keep it away from your pumps intake because small peices can damage the impeller. A wide variety of chemical filtrants are available to the aquarist. Through absorption, adsorption, flocculation or ion exchange, these materials remove compounds that are found to be detrimental to the overall health of some aquarium animals. Conversly only some water pollutants can be measured directly by typical test kits.
Heres the catch. If chemical filtrants are used to lower nitrate and phosphate levels, how can they be certain that other untestable pollutants are also reduced? and that significant trace elements like iodine are not consequently removed also. Many are not selective as to what elements they remove. Finally there is the issue of cost, If you spend $3.00 in chemical filtrants to remove polluntants from 10 gallons of seawater, its easy to miss the point that 10 gallons of brand-new synthetic seawater may cost you only $2.00 to replace.
