Aquarium startups (research "ammonia cycle") are typically very rough on fish. It can take days to weeks for beneficial nitrifying bacteria to colonize your substrate and begin the nitrogen process. Ammonia, being the first byproduct of organic material decay, is very toxic to fish. No matter what water conditioner you use, they won't stop its production.
Get a couple gallons of water from an established aquarium to "seed" your ammonia cycle after you've dechlorinated your initial water. Start checking the water quality daily. pH, ammonia, nitrites, and nitrates are the key parameters you need to keep tabs on.
Ammonia will spike, then fall as nitrites increase, then the same will happen into nitrates. Once this occurs, your tank is established with good bacteria and you should be good to stock it up, just not too fast as to overwhelm the bacteria either. They will adjust their population to the bioload of your system.