HELP! Beta looks ill

[Okami]

I was doing my weekly cleaning routine with the beta tank when I found out I had forgotten to place out some water. ( I leave tap water out for 2 hrs before using it in a fish tank) I left my beta in a seperate bowl with his usual tank plant. When I cam back after 2 hrs I noticed my betta was awfully pale, and very very frightened it would swim around the bowl non stop if there was a slight movement. I placed it back in it's tank, and it's now sititng at the very bottom among some rocks. What's wrong with the little guy? I've never had a problem like this before, the fish looks so pale you can see through his fins.
Earlier today the fish was fine, rich color in the fins and casual activity, if you even tapped the tank he wouldn't react.

[Sneaky]

Hi there,
did you add dechlorinator to the water
before you put it in the tank?
Your betta seems to be demonstrating a
negative response to the water change,
likely because the water was the wrong temp,
or had chlorine in it.
Chlorine and Chloramine in tap waters can burn gills
and kill fish on contact.

[Okami]

Yes I added both into the water and I let it sit for 2 hours before using it just so that it can dilute out. My fish is back to it's normal perky self. I think it was spooked because it was left in darkness in a bowl for 2 hrs then I suddenly came rushing down took out the plant he was hiding behind and plopped him back in his tank. I didn't know fishes sould change colors so fast.