Tap water is convenient and inexpensive but you probably don’t want the chlorine added to an aquarium and if you’re adding a lot of new water such as the 50% water change in an estimative index fertiliser scheme this becomes particularly important. Accordingly it is pretty standard to use a water conditioner to neutralise nasties in tapwater that might be present. I use Tetra’s AquaSafe and have never had any problems. A full capful (note: the outward-facing top part of the cap is the dosing measure, not the inwards-facing larger part of the cap) is 10 ml which will work for 20 litres of water.
Note the ‘in tapwater’ piece: the product doesn’t magically fix aquarium water that has already wonked-out chemistry. If aquarium water chemistry is a problem, you need to fix the underlying cause rather than attempting to treat the issue, which water conditioner won’t be able to do successfully anyway.
Having done a 50% water change with the water conditioner if I need to add a little extra water, maybe up to 200 ml, to top things up afterwards I don’t bother adding water conditioner to that extra water on the thinking that there’s already lots of water conditioner in the tank now and with good mixing there won’t be any meaningful extra stress that would merit faffing around with adding water conditioning agent to small water volumes.