why is my fish coverd with white spots?

this is not the first time that my fish had this happen but i changed my water/carbon also I checked the heat its on the right temperture and i feed them once a day now what can it be?
9 camel back shrimp
4 red leg hermets
5 domino damsel
1 blue damsel
4 stripe damsel
1 clark clown
2 tomatoe clown
 
Top