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Why you should never use used tank water to clean filter media.
If we are to discover the reasons why you shouldn’t rinse filter media in used tank water we first need to go over why we do water changes to begin with. Performing regular water changes has always been part of keeping an aquarium healthy and part of routine maintenance. This is without question something that most experienced aquarists can agree on. So why do we do it?
To remove detritus and decomposing uneaten food particles from the tank, which cause ammonia. Decomposing organic material releases nitrogenous products and other chemicals back into the water. We also do it to remove and help prevent the build up of nitrate. While fish are most times tolerant of higher nitrate levels, higher levels cause the fish to stress. Water changes help reduce this build up and keeps the nitrate levels at a healthy balance for the nitrogen cycle to perform effectively.
When you do a water change and all of that nasty water is sitting in a bucket you have to remember that the nastier stuff has settled to the bottom of the tank and inside of the substrate. Gravel vacs help remove those particles and at the same time removes a good portion of what was released back into the water, into the bucket. It’s not just the physical detritus that goes into that bucket. It’s the nitrogenous products and other chemicals that were released into the water column from the detritus and decomposing organic material. The soul purpose of removing that water is to maintain a healthy tank…. so why would you rinse your filter media in the exact same unhealthy water you are removing from the tank?
It’s always been a popular opinion to simply rinse filter media in used tank water because it’s the “easy” thing to do. Some say it’s to prevent contamination. I don’t follow this line of thinking, because clean dechlorinated water is going in the tank anyway and gets pushed through the filter media. I believe this idea has been perpetuated and passed down to new aquarists and passed off as the right thing to do simply because it’s “easy”. Look how long it took even the most knowledgeable aquarist to learn that ICH does not lay dormant in a tank and attack only sick fish (only an example showing how misconceptions get handed down and spread around)? Sometimes things that are easy are not always a good idea. It is my hopes that eventually people will see that using dirty tank water is not the best thing, and to take the extra few minutes to fill up a bucket of clean water treated with dechlorinator to rinse dirty filter media.