I add dechlorinator (Prime) to my water butt (90 litres) of tap water, before warming and pumping into the tank.
You can add dechlorinator directly to the tank, but....
- Add dechlorinator BEFORE you add tap water.
- Dose for full tank volume (not water added volume).
- Turn off filters and pumps etc first.
There are many stories here (and other forums) of people NOT dechlorinating their water and wiping out £100's of fish at water change time due to change in chlorine/chloramine levels in their tap water.
Also do not rely on 24hour chlorine degassing (especially in UK) as often in emergencies chloramine is added to the water and this is not degassed, a proper dechlorinator must be used.
If you are unhappy about paying Prime prices (despite the fact it can dechlorinate 20,000 litres), you can always use sodium thiosulphate. 1Kg from a certain auction site will dechlorinate 50,000 litres for £6. So absolutely no reason not dechlorinate water for your tank.