Even though lots of people simply add tap water directly to their aquarium and add some dechlorinator to the tank at the same time, there is always a chance it will go wrong.
Dechlorinators will neutralise chlorine molecules as soon as they come into contact with them. However, it can take time for the dechlorinator to come in contact with all the chlorine molecules and neutralise them.
Chloramine is a mixture of chlorine and ammonia. The dechlorinators break the chlorine/ ammonia bond and neutralise the chlorine, leaving free ammonia in the water. Some dechlorinators will also convert the ammonia into ammonium for a period of time (about 24 hours), which makes it safer for fish. The bacteria in the filters theoretically convert this ammonium into nitrite and then nitrate during that 24 hour period and stop it affecting the fish.
If fish swim into the chlorinated tap water they can be exposed to chlorine or chloramine (depends on what your water company adds to the water), and this can burn them and their gills. It also takes time for the dechlorinator to come into contact with all the chlorine/ chloramine molecules in the water. This means the fish are potentially swimming in chlorinated water for as long as it takes to get the chlorine/ chloramine neutralised.
If the water company adds extra chlorine/ chloramine, which they do during hot weather or after they have done work on the pipes, you can add tap water that contains a lot more chlorine/ chloramine than normal and wipe out the tank. I have done it myself and people come on here all the time with the same problem. There is a spell of hot weather, the water company increases the chlorine/ chloramine level. People do a water change like normal and fill their tank. the fish start gasping and die. They added highly chlorinated water to the tank and poisoned the fish.
Water companies do not normally inform you they have done work on the pipes and only inform you if there is going to be a major disturbance to the water supply. They can do work on the pipes any time, and will usually add 3-10 times the normal amount of chlorine/ chloramine to kill anything harmful that might have gotten into the water. A normal single dose of dechlorinator will not neutralise all of this chlorine/ chloramine and if you add heavily chlorinated water to an aquarium with livestock in, you will probably wipe out the tank.
This is why I always say any new water should be free of chlorine/ chloramine before it is added to the aquarium.
Ideally you want to fill some containers with tap water and add enough dechlorinator to treat each container of water. Then vigorously aerate the containers for at least 5 (preferably 30) minutes, or longer, before adding that water to a fish tank. This allows the dechlorinator sufficient time to come into contact with all of the chlorine/ chloramine molecules and neutralise them. The vigorous aeration helps by mixing the water and dechlorinator so the dechlorinator can come into contact with the chlorine molecules. (You can use a small water pump and hose to pump the water from the containers into the tank, if you have a large aquarium).
If you want to see how this works, get a large container of water and add a few drops or coloured food dye. Let it sit for a few minutes and see how much of the dye has spread through the water. The dye represents the dechlorinator and the water is the chlorinated water. If you stir the water up, the dye mixes in with more of the water, but it needs a lot of stirring to get all the water to change colour. Aeration does this quite effectively.
The aeration also helps get the dissolved gasses in the water back to normal levels. Water normally contains nitrogen, oxygen and carbon dioxide gas dissolved in it. Some or all of these gasses can be forced of the water while it's under pressure and aerating the water for 30 minutes or so allows the dissolved gasses to get back to normal levels before the water is added to the aquarium.
The pH of water can also change when it is aerated. This is normally due to excess carbon dioxide (CO2) being bubbled out of the water and oxygen taking its place. Carbon dioxide is an acidic gas and lowers the pH. If there is a surplus of CO2 in the water, the pH can be lower than normal and will usually go up when the excess CO2 is removed by aeration.
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not sure what you mean by this???
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Fish and plants need at least 8 hours of darkness to rest. They can have light for up to 16 hours a day, but they need 8 hours without any light except perhaps a bit of moonlight during the full moon. But don't try to replicate that in an aquarium. Just turn the lights off at night and let the fish and plants have a period of darkness where they can sleep and recover from the day.
