Safe Levels

Hi and welcome 

And well done on the fishless cycle 


1. I would drain the tank to the substrate, or if that sounds too much trouble, at least 75%. You want to add fish when there is just the amount of nitrate in your tapwater and the cycling process will have been making nitrate you need to get rid of.

2. I will assume you have an internal filter on that sized tank, which you will turn off during a water change as it doesn't do the motor any good to run it out of water. Add a small amount of water then add dechlorinator. When you add the rest of the water it will swirl around mixing the dechlorinator in. Newly grown bacteria colonies haven't had time to fully embed into the newly grown biofilm so you do need to be careful of them. Once the tank has been running a good few months, the biofilm will protect the bacteria quite well even from any chlorine/chloramine that gets to them before you add dechlorinator.
And on the subject of turning things off before starting a water change, don't forget the heater too. Many will shatter if they switch on out of water.

3. This depends on how many fish you have, how messy they are (ie how much they poo) and whether you have live plants.
For a reasonably stocked tank with no live plants, start at 25% a week, cleaning the substrate at the same time. if you have gravel, this means pushing the siphon tube into the gravel to get the muck out of there.
For a lightly stocked tank with a lot of plants, you can drop down to 10%.

Let the nitrate level guide you once the tank is fully stocked. Measure your tap water nitrate and do water changes as big and as often as needed to stop it reaching 20 higher than the tap level. Start at 25% a week and test before the next water change. If you find it hasn't gone up much, do less than 25%.

4. Test for several days every time you add fish or do anything to the tank eg washing filter media or removing decor. And test if the fish start to behave oddly. Once you've had the tank several months, test when you remember or add new fish.
In a stocked tank, ammonia and nitrite should be zero. Nitrate should be less than 20 above the level in your tap water.

I'll see if it will let me. Last time I tried to modify that post, it wouldn't let me as it was too long! Though I think the new owner has relaxed the post length restrictions.