If you know your water co uses only chlorine you can buy cheaper than API.
For many years chlorine was the only thing used to disinfect the water supply. So all fish keepers needed to make a tank safe was dechlorinator which is usually sodium thiosulfate. You can buy this in bulk cheaply.
Sodium thiosulfate (Na2S2O3) can be used to neutralize or remove residual chlorine. It simply reacts and forms sodium hydrogen sulfate or sodium bisulfate which is an inactive salt.
Then chloramine was "discovered as being a better option in many cases. Chloramine is a mix of chlorine and ammonia. When one uses a plain dechlor on this, it breaks into its two component parts- chlorine and ammonia. But the chlorine is detoxified as well which leaves the ammonia. So the newest products became a dechlor which not only broke down chelorine and also broke apart the chloramine but also contained an ammonia detoxifier to handle that.
Here is the one problem with using only a chlorine detox product. It is possible that one's water supplier may switch to chloramine. They are supposed to announce this stuff but it is rea;l;y easy for most not to notice the change or even to know about it. In such a case if sudden;y the water coming out of your faucets contained residual chloramine. The result would be similar to your pouring some ammonia into the tank but knot knowing it.
If one is sure that their water supplier only uses chlorine and will ssend agvance notice if they decide to switch to chlroamine, there is no reason to use the more expensive product. I am very lucky in that I have excellent well water and do not use dechlor. But I have both Prime and Amquel on hand. I take the prime to fish events where i may have to use tap water and I use the Amquel after blrach dipping plants or algae covered decor so I can return these to the tank immediately.
As for the bacteria, the ones we want are designed to use NH3. They have receptors for it. Many also have a lesser number of NH4 receptors which allow them to use this less desirable form of ammonia. But they are less efficient using it.
Because of how the toal ammonia works in a tank, most of it is in the NH4 form. However, it is possible to remve all the total ammonia in a tank just by removing NH3 or NH4. That is because as one removes one or the otherm the water chemistry quickly return the balance that the pH and temperature of the water dictate.
The final piece in this puzzle are plants. These also use ammonia, but they prefer it as NH4/ Plants are able to utilize NH4 much faster than bacteria can use NH3. However, one can have a well cycled tank which contains no live plants *sabe maybe some algae). One the other hand no plantedauqtic environment will be devoid of the nitrifying bacteria. The plants actually host these on their stems, roots and leaves. This is also why one way to deal with cycling issues when there are fish preset is to add live plants.
If you want more info on how cheap is can be to use sodium thiosulfate, have a quick read here
http://www.jonahsaquarium.com/JonahSite/dechlor.htm
