Fish keeping myth #1 – rinsing sponges in tap water will kill beneficial bacteria


Rinsing sponge filter in tap water. Photo by the author.

As I alluded to in the post on March 31st about starting a fish keeping myth series, I’m beginning it today. 

Fish keeping myth #1 is that rinsing your biomedia sponges in tap water will kill your beneficial bacteria. Not sure where this long standing myth began, but it is just that – a myth. Yes, you can rinse your sponges in tap water. For whatever reason, I see this all of the time. Novice fish keepers giving other novice fish keepers bad advice by telling them not to rinse their sponges in tap water. The claim is that doing so will kill the beneficial bacteria that resides on and within the sponge. The simple fact is, giving your biomedia sponges a quick rinse in tap water will NOT destroy the beneficial bacteria, certainly not enough to cause any kind of problem.

To have a deleterious effect on your sponge’s bacterial colony, the tap water concentration of monochloramine would have to exceed what is normally added to municipal tap water (in the United States). Most municipal water in the states do not exceed 2ppm concentration. Even at that concentration, one scientific paper suggests it would take over three hours of constant exposure to the tap water to destroy 99% of the nitrifying bacteria. 

Not convinced? See the 8th question in my interview with long time fish keeper Ted Judy. He’s a veteran hobbyist and even makes trips to CA/SA to collect fish specimens. Also check out this video by Jason Adams at Prime Time Aquatics. Still believe two seasoned fish keepers (three if you count me) are wrong? Then maybe Stephan Tanner, owner of Swiss Tropicals, who also has a PhD in Human Molecular Genetics, will convince you. Go to the FAQ page of his site and scroll down to the question, “How do you rinse Poret foam sheets?” 

If you want to read the science behind this, see the paper below.

Reference: Cunliffe, D. (1991). Bacterial nitrification in chloriminated water supplies. Applied and Environmental Microbiology, 3399-3402.

3 thoughts on “Fish keeping myth #1 – rinsing sponges in tap water will kill beneficial bacteria”

  1. Hi, very interesting but…
    “On thebasisof theresults,inactivationof99%ofanaturalpopulationof nitrifyingbacteriawithan average doseof1mg ofmono- chloramineperliterwouldtake760min.IncontrastWolfeet al.(13)foundthat99%oinactivationoflaboratory-cultured bacteriawasachievedin3to33minoftreatmentwith1.0mg ofmonochloramineperliter,”

    (Sorry about the lack of spacing, the copy-and-paste function caused that).
    Would you class an aquarium filter as a cultured medium or a natural population?

    • Hey Mark. Thanks for reading the blog and the note.

      I think that is an interesting question. In other words, should we differentiate a clinically cultured population from a natural population? With respect to the hobbyist, I’m not sure a distinction is important. To the best of my knowledge, there are no equivalent clinical experiments involving aquarium sponges, so monochloramine exposure effects are anecdotal in that context. What I will say is if the species of bacteria are the same (and in same quantity) in both the cultured medium and the natural population, I think you could infer that the effect of monochloramine on the cultured medium “should” be similar to the naturally populated medium. That is unless there is some mitigating factor about the media that differentiates the results (i.e., the “environments” and the media themselves are certainly different).

      It is a very interesting issue, and I’m sure some microbiologist hobbyists would love to dive in to this. Nonetheless, there are too many experienced aquarists who rinse their filters in municipal tap water (containing monochloramine) with no discernible change in their water parameters afterward. I will say that the population of bacteria in these filters should be mature, healthy, and ubiquitous within the filter. I can see where a young, small and/or immature population could be less robust and thus less capable of surviving a rinse satisfactorily to prevent an ammonia spike. That’s just speculation on my part, though.


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