I do not recommend adding water to the tank that has not been de-chlorinated, had the chloramine removed, and is not at the same PH and temperature as the tank. It should be aged at least 24 hours with a bubbler and heater.
That said, now the caveats and rambling on:
I have added water directly to my tank before. I would not if it contains chloramine. I am not very fond of the idea of doing it with chlorine. In my experience aged water at the right PH and temperature is the best. I had descent water in the past but my water now is so hard I have to use an RO/DI system and add Kent Marine River Right and discus
Essential. The discus
are healthier than they ever were on the aged and de-chlorinated tap water.
I highly recommend a plastic trash can with a heater and a bubbler. If you only have chlorine, no additive as long as your PH is fine and you bubble it for 24 hours. If you have chloramine, use something to get it out. Over time it will harm your Discus. It will also load your bio filter and make the new water old fast.
Chloramine is so nasty even with chemicals to remove the chlorine it was like adding urinal water to the tank.
When the chlorine molecule is released from the ammonia you have a new source of ammonia, your make up water. The only way to get out all the chloramine is with a carbon block filter. The ammonia can be removed with a very good carbon filter. It sounds like you may not have these problems since you have just been using tap water.
Our city changed from chlorine to chloramine and from aquifer water to surface water. Did they bother to tell anybody? Nooooo. I lost several Discus over a three or four month time frame until I figured out what they had done and went to the RO/DI system. It has been almost a year since with one death that took six months but never recovered from the illness the bad water caused so I credit his death to the city as well.
I got a utility pump for two reasons, if my main sump pumps fail, I have a back up, and to move water from the trash can to the tank. I have a permanent 1" vacuum hose with ball valves in the end. The exhaust has a trap on it so it does not lose prime when I shut it off. I even ran a PVC line from the prep trash cans to the sump. I can drain and refill 100 gallons in about fifteen minutes or less.