I have recently purchased a sponge filter just like this one. I added it to my tank when it arrived home about a week ago. My tank has been running for over 4 months now. The tank has been stocked for like 3 months, so it has cycled already. For now, both filter are in the tank to be sure I don't create an ammonia spike by removing the old hang-on-the-back filter.

My question is: how long should I keep running both filter to be sure the new one has all necessary bacteria in the new one? So I can remove the old one (hang-on-the-back).

  • Best bet is to not actually remove the filter, but to rinse it clean under warmish water.. ie water that is as close to the aquarium temperature as you can get. Why? To keep the active bacteriological filtration active. Do not use any soap or bleach or anything, just plain water. Preferably water that has been sitting in a container long enough to remove any chlorine. I have been running aquariums for over 30 years, believe me, the active bacteria in that filter is much more important than a clean new one. However it SHOULD be rinsed every month or so. Oct 30, 2015 at 0:05
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    @user3728148 as written in the question I want to remove the old filter. I know about the bateria hence the question
    – Rémi
    Oct 30, 2015 at 1:53

1 Answer 1


3 weeks or so should be enough to get some of the bacteria to transfer.

Note that only so much will grow in the NEW pump until the load is greater, so you will not get a full transfer from existing media regardless of how long you wait as the bacteria will stay alive in the media until it is removed.

So, if you have already had the new pump connected and already cycled for 3 months it would be a non-issue to remove the old sponge, but you may still get a spike regardless depending on how much the bacteria in that sponge are actually contributing... but I wouldn't worry too much about it. IT should be a very minor change at this stage, and since bacteria will have already transferred into the new pump, it will only take a couple days at most for more to grow to fill in for whatever is lost when you remove that sponge...

I just realized you are going in opposite direction, that is removing HOT (hang-on-tank) for the sponge. Please note that no sponge will ever provide the depth of filtration you would get from a HOT type filter, regardless of size of sponge... unless you are attempting to allow the fish to spawn. And even then, best I would suggest would be to leave the HOT but place the sponger over the intake to reduce the chance of eggs/spawn from being sucked into the HOT.

  • I finally decided to keep both as per your answer suggest. This allow to have a bit more fish in there without maintenance being more frequent than about once a week
    – Rémi
    Nov 30, 2015 at 18:04
  • That should be fine as long as you remember the following: Dec 2, 2015 at 22:45
  • That should be fine just remember these 1 Be sure the power of the HOT is not drawing too strong, don't allow it to trap the young in the sponge that you keep over the intake. 2 Remember young fish greatly increase the ammonia they generate as they are growing, meaning lots of small young fish, short in total length, create more ammonia than the same amount in inches - length of adult fish. So, don't go beyond min once per week maintenance. 3 live foods, best for young fish to grow, will also greatly increase waste by-products like ammonia and nitrates, I do wish you great luck with this! Dec 2, 2015 at 22:55
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    Please don't form your question like a chat message with "Wait... just realized that...". Edit it down to the relevant part only or ask a comment (but not one that has nothing to do with the question). Jan 14, 2017 at 18:38

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