Here's the third of a five-part series of lessons to guide new aquarists through the basics of fishkeeping.
Week 2, Day 3: Chemical filtration
Chemical filters use ion exchange or adsorption to remove pollutants directly from water.
Ion exchange filter media swaps one chemical in the water for another one. A typical example would be a resin that exchanges nitrate or phosphate ions with sodium ions. Water softeners also work on an ion exchange principle, swapping ions that contribute to GH or KH with sodium. When it has no ions left to exchange, and it is full to capacity, this type of media is said to be exhausted.
When exhausted, ion exchange filters can often be recharged for repeated use. Those that use sodium, for example, are often recharged using a strong salt (sodium chloride) solution.
Adsorptive media has a surface that is incredibly sticky to particular molecules. A typical example of an adsorption substrate in aquaria is carbon. Carbon will remove heavy metals and dyes from the water, though it does not adsorb ammonia, nitrite or nitrate.
Activated carbon is a type of carbon that has been treated to give it a higher surface area to volume ratio than standard carbon. The higher surface area means that it has a high adsorptive capacity.
When exhausted, adsorptive media like carbon is difficult to recharge (if at all) and is usually discarded and replaced.
The rate at which ion exchange and adsorptive substrates become exhausted is dependent on how much contaminant is in the water, and how much media is used. As a basic guide, it is suggested that carbon should be replaced every four to six weeks in a filter.
Zeolite is an important ion exchange (and sometimes adsorptive) chemical filter substrate that targets the removal of ammonia. Used in conjunction with a healthy, established biological filter, it helps to reduce the amount of ammonia being converted in to nitrite and nitrate.
If left in the aquarium for too long after it has adsorbed all that it can, a chemical filter substrate can eventually become a biological substrate, to the detriment of ‘real’ biological filter substrate. For example, because zeolite retains ammonia, it becomes attractive to bacteria that may eventually colonise it and ‘lift’ the ammonia from its surface to use as a food source.
How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.
Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.