Why do we change the water in our freshwater aquariums? Of course to keep the water clean from waste and ammonia derived toxics (NO2 and NO3). This is where hopefully all of us agree on when it comes to freshwater aquarium keeping. But the discussion starts when we talk about how often do we need to change the water in order to keep the readings low – and particularly with NO3, on what level.

On some forums, especially with visitors from North America, I hear a lot that I am crazy when I don’t change my water every week. I hear that I will get ammonia in my water, my nitrites levels will go sky high and all that. Well, my test kits don’t show me anything like that and I change my water every 4 weeks. Yes, not 1 but every 4 weeks. My test kits show me readings as follows: NO2 = 0, NO3 = 10-20, pH = 7.0. What would be the reasoning to change the water at this point?

I am not going to change the water, and by that affect the ecosystem, if I don’t get high nitrates or some nitrites levels. I don’t know if the water quality is just poor in North America (maybe not Canada, but how about California for example?) and because of that you have to change the water more frequently. But at least here in Finland we have very good tap water so this is not a reason to change the water.

Of course at some point you have to change the water even if the readings show normal numbers, since you want to gravel vacuum your tank. This would be the reason for me to change the water if the water value readings are ok. And for my tank the change frequency seems to be 4 weeks.

At this point it must always be said, that the water change frequency is always tank specific. Nobody can tell you how often you need to change the water in your tank. But please do that based on the water value readings and not based on some comment on some forum “Just because you should”.