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how long do I need to cycle a new filter for a established tank


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I got my new tank!!! :cheer:

well, the tank came with a second hand canister filter and I've installed it for my endler breeding/baby bristlenose grow out tank. The tank originally has a large sponge filter, the tank is established and has being running for a few months already. I've added the canister filter, but hasn't removed the sponge filter coz I'm scared that the bacteria number isn't good enough to deal with the bio load. So my question is when can I remove the sponge filter and leave the canister filter only for the established tank?

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I read somewhere on here that you should leave a filter in an established tank for a minimum of two weeks? It was a while ago and was just a off hand comment.

How long has the filter been in there? If it has been a couple of months then I am sure it should be ok?

But I am not a hundred percent sure.

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Ok... tank cycling 101 :) (though I don't understand fully what you mean... do you mean the tank is established and has been up and running with a different filter prior to adding the new filter? If you were running a new filter with an old one then the new one will be ready very quickly to use.. not much more than a week or two.. if this is what you are referring to and you are fine with the aquarium chem. stuff then I apologise for the following basic outline... I figured because you said it was a new tank that it couldn't have been cycled and things were starting from scratch again.)

You will find LOTS of different ways to do it. This is the way I was taught... 30 odd years ago at the ripe old age of 10 LOL Since then I've added a science degree to my understanding of it so it is now a little more 'fleshed out' ;)

You should go and get yourself 2 test kits; an ammonia test kit and a nitrite test kit. I don't believe the nitrate one is very relavent at this point, for this purpose, because a drop in nitrite implies conversion to nitrate which is less toxic (though good practice states it should be low and there is evidence to suggest it affects the growth of fry).

Stage One: Ammonia

When the water is added to a new tank the surfaces of everything in the tank, including the filter, will be devoid of aquatic de-nitrifying bacteria. It takes time for these populations to build up. The only way they canbuild up is if there is a source of nitrogen for them to metabolise (i.e. feed on). This can be something as simple as a flakes of fish food. What will happen first is the nitrogen source will begin to decompose and the be converted into ammonia. This is extremely toxic to fish so no fish should be added at this stage. You should monitor the ammonia levels until they reach its highest level or spike. At this point it will start to fall again because bacteria that feed on (metabolise) ammonia begin to grow and spread until the ammonia in the tank is all but gone. This usually takes around 2 weeks.

Stage Two: Nitrites

Once the ammonia is gone you are at the half-way point. The bacteria that metabolise ammonia produce a by-product as waste. They produce nitrite. This is also very toxic to fish, even at low concentrations. You should start monitoring nitrite levels and you will find that the levels will rise and reach a peak and begin to fall again. Different bacteria that metabolise nitrite begin to grow and colonise all the surfaces in the tank (note: the only reason we have filters in a tank is to provide a greater surface area on which bacteria can grow and colonise increasing the capacity of the tank to cope with the production of fish waste in your tank. They reach an equilibrium where populations of each component are in balance and any sudden dramatic changes can impact on this). After another 2 weeks or so the level of nitrite will fall to close to, or exactly, 0ppm and you can assume at this stage the ammount of ammonia is zero and the amount of nitrite is zero and there are sufficient bacteria to begin stocking your tank slowly remembering that increases in the amount of stock correlate with an increase in the amount of waste and a subsequent increase in the number of bacteria required to maintain the status-quo. You need to keep adding small amounts of fish food throughout the whole process to keep the nitrogen source going. At this point you can assume that all ammonia is being converted into nitrite and all nitrite is being converted into nitrate which is comparatively non-toxic to fish at low levels. You should have some form of plants in the tank because they take up the nitrate and so reduce the available nutrient in the water for alage. Higher plants can outcompete algae if they are growing strongly (I recommend fast growing plants like various forms of hygro, elodea (cough), and even duckweed which has enormous potential for nutrient harvesting).

So... if your tank has been going for a month and there has been a nitrogen source you can assume that the tank is now cycled. You should test this with both the ammonia test kit and the nitrite test kit to confirm this. If there has been no nitrogen source in the tank I would not assume it is cycled.


Take a filter sponge from an established tank and place it in the new tank. The bacteria will quickly colonise all surfaces and your tank won't cycle. I keep spare sponges in tanks or ponds for this purpose.

Have some Prime on hand in case of emergencies. We use to call this stuff 'life in a bottle'.

Don't go overboard with the water changes until after the nitrate phase is reached. You will need to perform frequent water changes to reduce nitrate levels unless you have lots of plants (note.. it IS possible to have lowly stocked unfiltered tanks IF you have lot sof very strongly growing plants). You can use chemical nitrate/phosphate scrubbers but I don't advocate these because I believe time spent learning the basics is time well spent and money saved. IMO using these things by-passes that understanding phase of learning aquarium chemistry.

Build stock up slowly to allow the bacteria time to adjust.

Be careful how you clean your filter sponges. They should NEVER be 100% cleaned. I like to time filter maintenance with water changes. Remove the water you want to change into a bucket and then rinse your sponges or other filter media in the old water and return it straight away. Never completely clean out a tank. Never take the sand/gravel all out and completelty clean it. Doing these things will only reset the cycle. Keep the substrate clean but never take it out and clean it thoroughly. Bristlenose poop a LOT... there will be lots to feed the bacteria... you should clean this out if it builds up but don't go overboard... it's just not needed and is just not smart. Plants LOVE it.

Hope this helps... there are lots of little tips and tricks one can use but this is the BASICS and now I don't ever do water tests... because your tank will tell you what is going on if you take the time to learn the signs. :)

Edited by TasV
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