But how ever long it takes air to change through our influence, multiply that by 100+.
Think of heat for example.
When your place is too cold, turn the heater on or light a fire on the lounge floor (depending how desperate you are) and it is warming up nicely within the hour.
Lets take water in the same volume as your house and try to heat it up in an hour.
So yes testing testing testing testing testing testing before you do anything at all.
Remember that what ever you do to the water now will take time to have an effect.
Even adjusting the pH (depending on gH and kH) will take time.
Test the water you are about to put in! You spot your little friend looking glum in the corner and go...
"Quick doctor give him a shot of new water that is just loaded with Nitrate and Nitrite, without a doubt that will remedy the situation! Stat Stat Stat!" Then you notice no change and think, OK the first diagnosis did not work, so lets put some more food in the tank.
Now adding to even more Ammonia and chewing more Oxygen.
OK Ok that is obviously not working.
"Lets test the water doc! What a brilliant Idea Watson! You will get your readings and because of all the previous dallyings, the readings will tell you absolutely nothing about your original problem.Tipping water in just is not the answer and should only be used after testing and you are certain you have a crisis that is time dependent.
Even better you now have another set of problems to deal with.
Most exposures to such things as Ammonia and Nitrite will take more than 92hours to take effect, unless you have outragious spikes of over 5,5,100 in a high pH solution.
Only total lack of Oxygen in the water and chemical toxins from outside of the water will require immediate action.
What can you do in this situation? Firstly calm down.
Recheck the readings to be absolutely certain there is a problem and do the test correctly.
The Nitrite, which is a salt of Nitrious Acid which is HNO2 not NO2 because it is dissolved in water and bonding with Hydrogen ions (not good).
This bonding to Hydrogen ions lowers the amount needed to maintain normal or neutral pH so as a result your pH should read low (acidic).
If the pH is low leave it there and definitely do NOT add water.
It is the sudden crash in pH that will kill them before the Nitrite makes it difficult to metabolise oxygen as a result of lots and lots of free radicals running about in the system called (RNS) Reactive Nitrogen Species these are a group of radical nitrogen-based molecules and one of these lovelys is Nitrous Acid go figure .
Then what your crayfish suffers from is called Nitrosative Stress which leads to nitrosylation reactions in your crayfish that will alter protein structure thus inhibiting normal functions such as Oxygen Metabolism.
You can also have the opposite Oxidative Stress from great little radicals like nitric oxide but that is not the case here.
Just want to note that crayfish are NOT fish and do not have the same sensitivities that most fish do.
You will find them a great deal more tougher than fish.
Neutralize the acid first if you get readings of Nitrite over two consistently for 2 days testing morning and night so a total of 4 tests before you do anything.
NaHCO (sodium carbonate or bicarbonate salts) will neutralize most weak acids in water, to where they will still be present however no longer negatively charged, so to speak.
The remaining Nitrite will still read on your Nitrite test which means the nitrifying bacteria can continue their normal function as you have not remove totally what they like to feed on.
The real test is in your pH.
If your nitrite and nitrate is reading high, your pH must read low.
If the acids are neutralised and still read but your pH is neutral or high then the job is done and the nitrite presents no further threat to the crusty.
Not sure if I explain that very well as it is my tendency to wonder off on tangents when talking chemicals and reactions as they never really stop.
The conclusion is, if you add new water you will add more problems and even more reactions.
Water takes time to move than air so changes you make are not instantainious - take your time and test test test.