I recently saw a post on an axolotl group where someone had shown water testing tubes with very high nitrates. The advice she was given was to do a 100% water change. I commented to please not do that as it would crash her cycle, but sadly it was too late. She had taken out 100% of the water and put in entirely new water in the tank. I told her that sadly the information she had been given was incorrect and that doing this would crash her cycle and suggested that she retest daily as she will need to take her axolotl out ASAP. Sure enough, the very next day the ammonia, nitrites and nitrates were all high again, showing that the cycle had crashed.
What does is mean when I say “the cycle crashed”?
If you’ve read my post about cycling a tank, you will be familiar with the term cycled. When the cycle has crashed, it means the beneficial bacteria are no longer there to do the work needed to keep the cycle going.
Why did this happen?
Once your cycle is established, you should only need to change 20% or your water weekly, but if you’re levels are looking a little high, you do need to take out a little more. However, rule of thumb, never take out more than 50% of the water at once. Doing this will crash your cycle and you will be forced to start over again.
What if your Nitrites and/or your Nitrates have maxed out and you do need to take out more?
This chart shows the recommended amount of water to take out based on your readings. You will notice that the higher readings do say to take out all the water. However, the 50% at once rule still remains the same.
If you have an axolotls, tub it immediately for this process, if possible for fish, do the same, but its not always possible for a community tank.
If you are taking out more than 50% of the water, from the top, don’t vacuum the substrate. Start with 50% only and replace with clean primed water, the same temperature as your existing tank, wait 2 hours and take out another 50% and replace. You may need to do this 3 or 4 times to bring your levels down. After 2 round you have 25% old water, plus healthy bacteria, then 12.5%, then 6.25%. Doing it this way will ensure you are not crashing your cycle and you are not shocking your bacteria or any fish if you have had to leave them in the tank.
How do you know your cycle has crashed?
If you’ve made this mistake and crashed your cycle, you will have to start the cycling process again. However it should be as long as before as some bacteria should survive to start a new colony.
Are there other ways to crash your cycle?
Yes, the most common way to crash your cycle is incorrect filter maintenance. Some people are advised to replace their sponges every month. Don’t ever do this! Most of your beneficial bacteria live on the filter sponges, throwing them away means there aren’t enough bacteria in the tank to keep the cycle going. See Filter maintenance for further info.
Someone told you it was ok if the temperature, pH and other parameters to match the water you are replacing?
In theory, yes, if you match everything exactly if will be fine. An aquarium shop for example who have stores of primed water and keep their shop at the correct temperature, will have water matching what’s in there tanks. However, very for people have this in their homes. This means its extremely hard to achieve this exact match for a normal person and in my opinion, not worth the risk when there is another way.
