Hyper Zebra Danio Suddenly Mellow

[SakanaLover2009]

Hello there, about a week ago I purchased three zebra danios and put them in a 78 degree F ten gallon fish tank. The fish tank is slightly populated with plants which I have noticed they like to swim around and zig zag and do mainly tag which every zebra danio enjoys! Well I have had this one zebra danio was is bigger than the rest and very very playful. I come home to find him depressed looking, at the surface of the water, and just not acting himself.
So I come to ask you mainly put this in emergency since I just had a fish expert come look at him and not even she knows what is wrong. After first we thought it was dropsy since he is a big fellow but apparently not that big. He isn't showing any outwardly symptoms such as discoloration or popeye. He just is acting depressed.
He hasn't given completely up though. I noticed while waiting on my fish friend to bring me meds that now I know won't work that he will still swim to food and eat and will run if his tank buddies decide to play tag which is better than he just standing there and taking the tease.
So I was wondering if you could help me out please! I don't really want to lose him! The pH level is at a decent level which I know that isn't it....so I don't know what it is. If there are any new symptoms I will be sure to post them here. Thank you

 

[WILDER]

What are your water stats in ammonia, nitrite, nitrate, and ph.
How long did you climatise the new fish for.
What was the ph of the store to your tank.
Soemtimes it can take a while for fish to settle in.