good water??? signs of stress???

I have a 20-gallon salt water setup here it is 2-penguin 125 bio-wheels 1-30 gallon capacity powerhead with aeriator(for water circulation and aeriation). 1-air stone 1-fully submergeable heater. Now the tank has been setup for 3 weeks.I added the salt water and cycled for the first 24hrs then added 1 bag of cultured crushed coral substrate. waited 48 hrs then added an 8lb fiji live rock then started adding fish first i added 2-tiny clowns then 1-blue damsel waited a week then added 1-yellow wrasse waited another week then yesterday i added 1-choc chip star 1-turbo snail and 1-cleaner shrimp which died overnight(only the cleaner shrimp died).Since the start all my levels have been excellent!! water is steady 78 degrees ph-8.2 , 0-0.25ammonia , 0 nitrite , 0 nitrate and a salinity of 1.020 water is crystal clear. Now heres my worry this is all new to me saltwater that is but ive had brakish water fish for 10 yrs and i know signs of stress. Heres how my fish are acting they arent eating right ,but must be eating something, considering that the clowns are 2 weeks old .Im giving them marine flakes, brine flakes and frozen brine.Im seeing heavy and rapid resporation,and lack of activity.Please any advise is greatly appreciated thank you for your help your pal HURRIE UP!!