How to "cycle" a biological filter

The most often asked question in our shop when somebody is new to aquatics is "how soon can I put some fish in the tanks ?" When you say six to eight weeks people look at you as if you are daft.

Now many of you will know all about the nitrogen cycle and the importance of a mature filter to deal with the waste our fish produces, but for those that don't I will explain it anyway, and I am not going to get all technical either.

The basics of the nitrogen cycle are AMMONIA (NH4)to NITRITE(NO2) to NITRATE (NO3), this happens be it a few Zebra Danio's in the tank or twenty adult Discus.

Ammonia is highly toxic to fish. It is produced and accumulates in the water when the fish feed and from any decaying food or plants etc. Ammonia will irritate the fish and eventually kill them or short term stress them to the point where they will become susceptible to white spot, fin rot and so on. If the water has an acidic pH you will have Ammonium (NH??) , which is actually less toxic, but none the less undesirable.

Nitrite is less toxic, but needs to be "zero" for perfect water conditions. If nitrite is present in the water it will poison the fish as it enters their bloodstream and deprives them of oxygen.

Nitrate when "rising" is a good sign as it confirms the biological filter is working, but high levels can lead to problems also. So is best kept to a minimum and the simplest way to keep Nitrates low is by regular partial water changes.

So how do we deal with these contaminants and make our water safe for fish? For the purposes of this article I am assuming you are new to fish keeping and want to keep Discus. Also this is the method I use and it works for me. You may well find alternatives on the Internet and forums that work just as well.

A "fishless cycle" is the best as it involves no subjecting of livestock to ammonia and nitrite.

This is how you do it.

You will need an Ammonia test kit, a Nitrite test kit, a Nitrate test kit, some household ammonia and a good deal of patience. I would recommend individual liquid test kits, not the multi test strips you can buy. For the pH an electric "pen" will be more cost effective and convenient than a liquid test kit, but either way you will need to know the pH of the aquarium.

- Fill your aquarium with de-chlorinated water.
- Extra water movement / oxygen will help no end, so consider adding an extra air stone during the cycle.
- Fit your chosen filter and let it run for 24 hours. Your Discus dealer may let you have some "mature media" - if so add this to the filter as it will speed the process up.
- You can add various "cycling" products, following the manufacturers instructions. Add the correct dose 24 hours before adding any ammonia.
- Buy some "household" ammonia, this is a cleaning product and is available from most hardware stores. You may see "extra strength ammonia for fish tanks" on Ebay. Not required!
- Dose your water up to 3ppm of ammonia, there is a useful online calculator here..
- After seven days test the ammonia content of the water, it will if a "clean cycle" be at exactly the same level as a week ago, remember I did say you need the patience of a saint with this!
- Continue to test every other day until you notice the ammonia decreasing. What this means is that the nitrifying bacteria are starting to colonise the media and the Nitrite reading will increase.
-At this point this is where the way we do it differs from many others. As the Ammonia decreases to nil, I re-dose to 1ppm (not 3ppm) - the reason being that to dump 3ppm onto the bacteria is just so unnatural. We have found that 3ppm re dose can stall the cycle, whereas 1ppm does not.
- 1ppm of Ammonia should reduce to nil overnight, the Nitrite will build to 5ppm +, IE very red on the test kit and stick there for what will seem ages.
- Then almost overnight the Nitrite goes from very high to nil, at this point the Nitrate reading will increase indicating the cycle is complete.
- To test this, carry out a 90% water change and dose to 1ppm ammonia. If the cycle is complete then within twelve hours the tank water will read - Ammonia nil, - Nitrite nil, - Nitrate some.
- Your aquarium is now ready for some fish - lightly stocked to start with and you can then add more as the test kits confirm your filter is processing the waste properly.

- Tap water in the main will contain some Nitrate, so your aquarium will never be lower in Nitrate value than the water you add to it. Cycling a filter "properly" can take 21 to 28 days with the addition of a little mature media from a trusted source, but a "clean cycle" will take six to eight weeks, but is well worth the effort for the health of your fish.
- Cycling is aided when the pH of the water is alkaline ( 7 +)