Gasping Clowns

I recently (2 days ago) downsized my tank from a 30 gallon to a 20 gallon, and used fresh saltwater from my local marine pet store to fill it. My clownfish seemed stressed out at first, but calmed down over night. I have a protein skimmer from my 30 gallon tank on it, and i bought a new filter. I recently noticed that the clownfish are rapidly gasping, and behaving sluggishly. I know the water is aerated enough, and the new water shouldn't be tainted or anything. I'm hoping the cause is just stress or shock, but I want a second opinion. can the water be too aerated with such a large protein skimmer? Thanks for any responses.