Water changes during cycling?


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Dec 9, 2023
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SF Bay Area
I've heard arguments for and against water changes when cycling a new aquarium, so I'm not sure what I should do. I plan on including plants in my aquarium while it cycles, but no fish or other animals.

Additionally, I will likely not be adding any fish or other animals to my aquarium for at least a few weeks after the cycling process has completed. How should I handle water changes during this time?

Thanks in advance!
You can do water changes whenever you like during a cycle. They can be necessary in very soft water.
You can't interrupt the cycle once running... You must continue to add ammonia until you are ready to introduce your fishes. Do a Large water change before, And have the shortest delay possible in between the direct introduction of your full fish stock.

Without doing so... Your tank is going to re-enter cycling process on introduction... These little babies die fast.

You can do water changes when Nitrates rise too much. If your water supply is a little on the soft side. I would also persevere as much as possible to create a soft water resistant colony.

You can do water changes as soon as you feel one of the nitrogen byproducts is too high. But in reality if ammonia and nitrite are steady 0... Nitrates is the scale to follow.
Sorry it's a professional deformation...

Large part of my job is to vulgarize and explain technical stuff.

I tend to elaborate a little too much...
Under normal conditions it should not be necessaary to change water during a fishless cycle done properly. However, there are reasons it may become necessary to do a water change. Both too much ammonia or nitrite bac stall a cycle. For most, not following the method offered here, is the reason why. If one allows ammonia-N to exceed 5.00 ppm (tusing the Nitrogen) scale, or about 6.4ppm using the Total Ion scale (most common in the hobby), one can stall a cycle or colonize the wrong strains of ammonia bacteria. Morover, exceeding 5 ppm of nitrite-N (nitrogen scale) or about 16.4 ppm on the Total Ion scale.

If one exceed the above numbers you will need to do a water change.

Next, the bacteria us inorganic carbon. The can get this a few ways. One is from CO2. Another is from the things that mostly make up the KH in tanks- carbonates and bicarbonates. These are also what holds up pH. So, if one starts with a somewhat low level KH. it can get used up which will result in a drop in the pH. And when the water gets much below 7.0, almost all of thr ammonia is in the form of ammonium. The bacteria can still use this but with much less efficiency than ammonia. So, there must be more of them to clear any given level of Tptal Ammonia (the sum of ammonia NH3 and ammonium NH4). This will basically slow a cycle and it may even appear to have stalled.

If the above occurs the normal solution is to do a water change.

If a fishless cycle goes wrong and the reason is not clear. The best solution rather than trying to fix it would be to do a big water change and then to resume from the last point before things went wonky. The key to a cycle is ammonia- it all starts there. So this is how we control the process of cycling.

Next, you can hold a cycle ona tank for some time simply by adding ammonia. Assuming you have completed a fishless cycle and the tank is ready to receive a full fish load, you can postone this if needed. Most of the above is dealt with in the article on this site on fishless cycling.

A major benefit of this fishless cycling method is that you can now fully stock your tank in one go. This means an average stocking level for your tank size. It certainly does not mean you can stock heavily or over stock. If for any reason you are unable to stock the tank when it is cycled, you can continue adding ammonia to keep the tank cycled.

The bacteria do not need to be fed every day so you only need to add ammonia every 2 or 3 days. If the wait time to add fish will not be more than a week, you can just add the snack dose amount 2 or 3 times in total and things will be fine. However, if the wait time to begin stocking will likely be longer than one week, do not use the 1/3 snack dose, instead, use the full ammonia dose you did when cycling. You can do this for many weeks if needed.

Keeping a tank without fish fully cycled for extended amounts of time requires doing weekly 50% water changes. Do these on the days you plan to add ammonia and do the change before adding the ammonia. If you do not do weekly water changes, nitrate will build up and KH levels will drop which can cause unwanted issues with the cycle. Further, other things the bacteria need may get used up and normally these get replaced by the new water going in. Of course, remember to do as big a water change as possible before adding fish, even 80%+ is fine.

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