My Betta, Neptune, lives in a BiOrb FLOW 30 tank which has a spongy filter material sitting on top of charcoal pellets.

I was changing it monthly but got behind and the current filter is going on 3 months. I also got behind on water changes (don't judge, I just had a baby) and, to my surprise, the water tests showed 0 ppm ammonia, nitrite, and nitrate and a pH that's 7.8, which is the same as the tap (so, it's been stable).

Given that everything is so stable, it feels like a filter and water change aren't necessary at this point, but that just feels so wrong.

So, the question is: given this stable environment, is a water and filter change necessary?


  • 1
    In addition to the existing answers: The water changes dilute products in the water which are not measured with the standard tests. Jul 17, 2018 at 15:00

3 Answers 3


No, you do not need to change the filter material. But what you need to do is remove the charcoal and replace it with new filter foam (filter sponge).

The charcoal is only needed if you are removing medication from the water.

When you can see the water flow from your filter is reduced you need to clean the filter foam; to do this, you take some water from your tank and use this to clean the foam from your filter.

You need to change 10-20 % of the water every week, this is to remove the nitrate that builds up in the water.

A waterfilter functions by giving the helpful bacteria that break down waste from your fish a place to live and work. So if you remove all of the foam from your filter, you remove the helpful bacteria, too.

The bacteria living in your filter converting waste to plant nutrients are called nitrifying bacteria and what they do is called the nitrogen cycle; more about this here could be found in this article about the nitrogen cycle.

  • The fact that I have 0ppm nitrate leads me to believe that I've got a great cycle set up - no reason to screw with it...right?
    – user12553
    Jul 17, 2018 at 6:15
  • you are right but you still need to do some water changes,and i suggest changing the coal with a filter sponge it will make the filter even more effective at handeling the waste. Jul 17, 2018 at 6:26

The filter sponge is where the beneficial bacteria live which help to take ammonia, turn it into nitrite and then finally convert it into nitrate. These bacteria can also live in the substrate (gravel, sand, dirt etc). In a well established tank, which is what is sounds like you have, your main concerns will be the nitrates.

There's a lot of debate about changing filter material. You don't want to throw away your existing sponge because you're basically throwing away your bacteria with it. Whilst you may find much of your bacteria lives in the substrate, you don't want to get rid of any more bacteria than you can afford.

To clean your filter sponge, take it out, squeeze it a few times in some tank water to remove detritus and then replace it in your filter. As Trond alluded to, you may wish to do this once you notice the flow rate in your tank drop.

The main thing I would be concerned about

In a tank which is only 30 litres, you need to ensure you keep a good water chemistry balance. With less water, a small change in the environment can have much bigger consequences on the fish. A larger system is much more stable which is often why you see beginners with small tanks struggling to balance their tanks.

I would double check your reading on Nitrate to make sure it really is 0. Admittedly, the bio load in your tank is not high, but just double check anyway. Consistency is key and ensure that you do your weekly water change will help your fish thrive. Just ensure that in a lower volume system, the water you're putting in is as close to the parameters in your tank as possible to avoid big spikes in your water parameters.

More generally

I have seen 'no maintenance tanks' thrive. This is predicated on the idea of balance and normally only works in planted tanks. The plants in the tank will help to keep your water quality high anyway by using the nitrates. This means that there is not really a need to change the water as long as the plants get everything they need to grow. The balance is often a little tricky but it sounds as though this could be what is happening in your tank. Even a 'no maintenance tank' should be monitored closely though to ensure that no levels become harmful to your fish.


I would say at 3mos. its now a good time to keep the good environment going. Yes when I had this type of setup, actually as well as the other 1-piece complete filters, I found ways of stretching them. As a kid with not much money, it hadda be done at times. But with all pets we wanna do whats right, right? So, as I said after 3mos. I believe a change is needed. But in btwn time or if u MUST go longer: change the spongy stuff (if its the kind that comes in a big bag of it) and help reactivate the charcoal by giving it all a good wash & rub under cold running water for several mins. Always make sure b4 u do anything with ur tank/fish u rinse off hands & arms well without using soap. And continue the good habit of regularly testing the water.

  • 2
    You're not really responding to the arguments of the OP. Jul 17, 2018 at 15:01

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