When can I add fish to my freshwater aquarium - the nitrogen cycle
The age old question… When can I add fish to my aquarium?
It’s only natural to want to put fish into your new aquarium right away. That’s why you got one in the first place right? So let's do it! Hold your horses there, partner. If you throw a bunch of fish in your tank right off the bat, it’s going to cause problems with water quality.
The quick and dirty run down
The reason you want to wait is because your aquarium starts off as a sterile glass box. You need to give it time to develop into an ecosystem, however it will never be a fully functioning ecosystem. You’ll still need to scrub the glass and do water changes. But, you will be able to create a balanced system by giving your aquarium some time to break in and develop as a miniature ecosystem.
The aquarium nitrogen cycle.
This process is called cycling your tank, and all you’re doing is allowing enough time for nitrifying bacteria colonies to develop in your aquarium so they can complete the nitrogen cycle. This is what turns your tank into an ecosystem. Nitrifying bacteria convert ammonia, a very toxic fish waste, into nitrites and then nitrates, which are a less toxic compound. Aquarium plants absorb nitrates, so you can essentially create a little cycle in your tank where fish poop gets turned into plant matter. Then, plants put oxygen in the water which helps the fish and everyone wins!
How long does it take to cycle?
It takes four to eight weeks to cycle an aquarium.
What’s the best way to cycle an aquarium?
One of the most important things to keep in mind when cycling an aquarium is that you want good places for the bacteria colonies to live. The best kinds of materials for that are porous ones like lava rock, clay-based substrate and ceramic rings that you’d put in a filter. Denser materials like river rocks and standard aquarium gravel won’t support nearly the same amount of bacteria.
Next, introduce some bacteria. I like to buy bacteria colonies like this, but you can get any type at your local store and it will do the job. Be sure to provide a source of ammonia as well. Live plants are a great source because plant debris break down and release ammonia. You could also use household ammonia, but you run the risk of over doing it - I’ve never personally gone this route but it is an option.
My favorite source of bacteria is substrate or filter media from an established aquarium. If you can fill your filter with media from an old aquarium it will disperse bacteria all over the tank. You can also mix some old substrate in with your new substrate to help spread it out and get a quick establishment of bacteria.
How do you know when your tank has finished cycling?
The only way to know if your tank has finished cycling is to monitor your water quality along the way. You can use test strips or a drop test kit to test your water. I used to be all about the test strips when I was younger because they are quick and they give you a decent ballpark picture. Nowadays I use drop tests because they are more accurate.
What you are looking for is a spike in ammonia followed by a spike in nitrites, then nitrates (Fig 1). You should see each compound rise and fall. This process usually takes a bit over four weeks to complete.
Figure 1. Graph of theoretical nitrogen cycle to illustrate how each chemical compound rises and falls over the course of 4-6 weeks. These are not actual values being shown. In reality, ammonia will show up in far less concentration than nitrates.
When can I add fish to the aquarium?
It’s best to wait for the cycle to complete before adding fish. If you do add fish to the aquarium before cycling is complete, you will likely subject them to high levels of toxic substances. Adding fish before the cycle is done will also increase the degree to which each compound spikes (ammonia, nitrites, and nitrates). This is because fish excrete ammonia into the water, and will add to the existing ammonia. The more fish you add, the larger the spikes you will see.
Take a look at what happens when you add a few fish while cycling (Fig 2). Instead of maxing out at a concentration of 25, ammonia, nitrite, and nitrate levels are now maxing out above 30. Let’s say theoretically, most fish can only tolerate up to 25. You could look at this chart and see where the ammonia level meets the deadly line at day 11. That’s when fish will start dying. So throwing fish in your tank right off the bat won’t kill them right away, but it will lead to a slow build up of toxins that will kill them in a week or two.
Figure 2. Theoretical graph showing how adding fish before cycling is complete increases the toxic chemical compounds in an aquarium.
Let's have a look at what happens when we fully stock a tank with fish right off the bat (Fig 3). Within seven or eight days, the water is already deadly for most fish.
Figure 3. Theoretical graph showing how fully stocking your tank right off the bat will make your water quality horrible!
How many fish should I get at a time?
Take it slow when adding fish to your aquarium. I like to stock about a quarter to a third of the fish I’m planning on putting in at a time. So if my goal is to have 12 fish, I’ll put four in one week, four the next and then four the following. This gives your aquarium time for bacteria colonies to grow with the increasing fish population.
Adding too many fish suddenly can cause smaller spikes in ammonia because your ecosystem of an aquarium is not accustomed to that bioload. Fortunately established bacteria colonies grow pretty quickly. So if we give them a little time, they’ll be able to catch up quickly.
Exceptions to the rule
As always, there are exceptions to the rule. You can almost completely skip over the cycling process if you use substrate from an already established aquarium. I’m talking about all of it. Not just a few handfuls. All of the substrate in the new set up must come from an old aquarium. Even then you’ll likely see a little spike in ammonia just from agitating everything.
I’ve heard people say you can bypass cycling by using a canister filter from an established tank. I think it will certainly help speed up the process, but you still can’t skip the step completely. There is so much more bacteria being housed in substrate versus filter media. And from the times I’ve tested it, I’ve seen smaller spikes in the three compounds - ammonia, nitrites and nitrates, but have never been able to completely bypass cycling this way.
Drop a comment below if you have any questions and stay tuned for more detailed water parameter blogs in the future!
