How to Mature/ Cycle a Tropical Fish Aquarium the Easy Way

Look after the water and the fish will look after themselves. That has been a mantra of fishkeepers for many years and one I also stand by.

The first step in maintaining healthy aquarium water takes place before any fish are introduced. That first step is to make sure you mature your aquarium filter. A mature aquarium (or pond) filter will quickly consume toxic chemicals in the water that are harmful to your fish (ammonia and nitrite).

You'll find many opinions and methods of how to do this, but I'm going to share with you a fool-proof method that will ensure your filter is packed full on beneficial bacteria which make for a safe environment for your fish.

Do this in a bare aquarium (no decoration or gravel) with both your filter and heater running. This method will also work in a goldfish aquarium, just remove the heater.

What You'll Need

Fish food
Filter bacteria culture
Ammonia test kit
Nitrite test kit
Nitrate test kit
Tap water dechlorinator

1. Seed the Filter

We want to build up a large colony of beneficial bacteria (Nitrosomonas sp. and Nitrobacter sp.) inside your aquarium filter. This process can be sped up by introducing a starter culture of these bacteria. Any good aquarium shop will have these available, often sold as 'filter starters'.

If you do not add a filter starter, this maturation process will still work! It will just be a slower (and longer until you can get your fish!).

2. Feed the Bacteria

To get your filter bacteria growing and multiplying, you'll need to feed them. The first bacteria we want plenty of is Nitrosomonas sp.. Nitrosomonas bacteria use ammonia as food. Ammonia is incredibly toxic to fish and is produced in aquariums by being excreted from the fish and from decaying organic matter.

We're maturing the aquarium without any fish in it (because we don't like being cruel to fish) so we need to add the ammonia ourselves. Now, in the past, I have added an ammonia solution. But, it's nasty stuff and not something I'm going to recommend. We'll be adding some ammonia by introducing some organic waste in the form of fish food.

As we have no fish in the aquarium, we don't need to be ultra-accurate in our dosing. Add roughly a 'pinch' per 50 litres of aquarium water. Do this daily until your ammonia test kit is reading 6ppm, then stop.

3. Take the Ammonia to 0

This is the easy bit. We're going to monitor the ammonia level in the water until it reaches 0ppm. After a couple of weeks (there's no point before as the ammonia will still be high) start testing the water every 2-3 days until all of the ammonia has gone.

This process usually takes 2-4 weeks, but all aquariums mature slightly differently, so don't worry if yours takes longer.

4. Take the Nitrite to 0

As the Nitrosomonas bacteria 'feed' on ammonia, it is converted into nitrite. Nitrite is not as toxic as ammonia but it is still bad for fish so we need the level of nitrite to be 0ppm.

At this stage Nitrobacter sp. bacteria are consuming the toxic nitrite and are multiplying their numbers. We need to test the water regularly and watch as the nitrite level falls to zero.

Important: whilst we are allowing our Nitrobacter bacteria to consume nitrite and reproduce, we mustn't let our existing colony of Nitrosomonas (ammonia 'eaters') starve. We need to keep adding some ammonia ourselves until we have fish waste feeding them. A small pinch of fish food per 100 litres of water every other day will be plenty to keep the Nitrosomonas ticking over.

5. Dump the Nitrate

Nitrate is broken down into nitrate which is only toxic at high levels. This maturation method can cause high levels as large amounts of ammonia, the nitrite are processed. Nitrates are removed by plants (which use it as food) or by water change, so, we're going to do a large water change.

The scale of this water change depends upon the fish you want to keep as some fish tolerate it better than others. As a general rule, try to keep it below 25ppm for most tropical fish although many can take it higher.

So, as an example, let's say that you test your aquarium water and the nitrate level is 100ppm and we want to get it down to 20ppm. You should also note that tap water contains nitrate, in our example we'll say that it comes out the tap at 10ppm nitrate. A 90% water change will drop the nitrate level in the aquarium to 19ppm. If you need help calculating your water change, leave a comment.

It's important at this point to use a tap water dechlorinator as high levels of chlorine in your tap water may negatively impact your hard earned friendly filter bacteria!

6. Decorate and Add Some Fish!

OK, so your ammonia and nitrite are zero and you've got the nitrate pegged back, time for some fish!

Because you've matured your aquarium in this way, you could (in theory) fully stock it in one trip to the fish shop. However, there are more factors to introducing multiple fish at once than just water quality, so I would not add more than a third of your final stocking level. You can gradually increase your fish every 2-4 weeks until you're full.

Questions?

This is a simplified guide based on what has worked for me over 25 years of fishkeeping. Maybe you'd like more details? Please ask any questions you may have in the comments below. The chances are that you're not the only one who wants the answer!