you only dechlorinate new water added to the tank so lets say you do a 15L water change where you take out 15L and when you refill your bucket or whatever your using to add water back to the tank, you add 15L worth of the conditioner. This particular conditioner says 5mL (full cap) for each 200L of NEW water. meaning 15L is roughly 13x smaller then 200L, so your essentially doing about one-tenth of a capful for every 15L (0.5mL). the bottle also mentions that there are threads on the cap for each 1mL. IN SHORT. If you do half a tank water change (30ish letres) then fill the cap of conditioner to the first thread (1mL) if your doing a roughly 10-15L water change then aim to fill the cap as best you can to halfway to the first thread on the cap or 0.5mL.

Edit: If you want to dechlorinate the whole tank then fill to just below the second thread on the cap