Chlorine in low concentration is still harmful to fatal for fish. Ammonia is way different. How bad it might be depends on other water parameters.
So,why do we add dechlor? It is supposed to be because of residual amounts in tap water. But what about ammonia which can be created when chloramine is broken down into its two parts- chlorine and ammonia? Consider first we are discussing residiual concentrations. For most of us who use municipal water there are regulations involved which dictate how much of what is permissible in the water supply. This is not very much chlorine or ammonia. It is certainly not enough to be a problem for most humans.
But fish are not human. We do not breath water. So the question, in terms of fish, becomes is the water safe in terms of chlorine and ammonia. The answer, in many cases is yes. But in the interest of better safe than sorry we have declor.
Lets not forget that in many tanks with a pH closer to neutral that ammonia in the water is mostly ammonium which is way less harmful. Next ammonia evaporates especially in when the water has the surface roiled which promotes gas exchange and allows Ammonia gas to evaporate.
If we want to we can even test this all. I can lay out a simple experiment most of us can do to determine how much residual ammonia is being created when we have chloramine in our tap and we use a dechlor which will break that down into chlorine and ammonia.
1. Put 1 gal. of tap water into a clean bucket. Test it for ammonia. If you have a test for chlorine do that as well.
2. Also, test the pH and the take the temperature of the water.
3. Add the proper (not extra) dechlor to the bucket and swish the water around to spread it.
4. Wait about 5 minutes and repeat the tests in step 1.
You will now know the concentration of Total Ammonia (NH3 + NH4) in the tank.
Now head on over to the ammonia calculator here:
http://www.hamzasreef.com/Contents/Calculators/FreeAmmonia.php
Then do this:
1. Choose NH (NH3 + NH4)*
2. Enter in the total ammonia reading from your test, choose ppm.
3. For a fresh water tank, enter 0 for the salinity.
4. Enter your tank’s current pH.
5. Enter your tank temperature and choose F or C, whichever applies.
6. Click Calculate.
The number you want to know is the one for
NH3.
[* If your kit measures ammonia as nitrogen aka –N, choose NH-N (NH3-N + NH4-N) in step 1. above.]
Why do you want this number? Because, to get a tank cycled you need to have ammonia (and nitrite) present and you want to have as much total ammonia present as possible without causing permanent harm to the fish. And it is the NH3 that will cause the real harm. Your goal is to allow the total ammonia to rise as high as possible as long as the NH3 content does not get to .05 ppm using that calculator.
Hint: As a rule, problems from ammonia become increasingly serious as one’s pH goes above 8.0 and by 8.5 one must really be doing a fishless cycle only. This is very relevant for those keeping rift lake cichlids
The above is from one of the cycling articles on the site. But it works perfectly for the test. You are interested to see two numbers for the water in the bucket. How much ammonia is there when it arrives from the tap, how much ammonia is there after using dechlor to separate the ammonia and chlorine. But what you really want to know is specifically how much NH3 is in the water.
Then remind yourself that in a cycled tank you have bacteria to consume ammonia. As soon as it is available, they start to use it. They even have the ability to increase how much they use by a bit before they start to reproduce as a result.
So unless yuo perform the bucket experiment and the answer is that you have a real amount of Total Ammonia, and then specifically NH3 above 0.05ppm, that ammonia will not be a problem and it will be removed by your bacteria and evaporation pretty fast. And if you are still worried remind yourself that, "Activated carbon is an adsorbent that can adsorb ammonia in the form of gas with a short contact time."
edited for typoes and grammar