I think even for a cycled tank 30 fish is a lot to add all at once. It's better to add one or two at a time because the bacteria have to have the time to catch up with the extra bio load. It could be that your water quality isn't good because of this. Are you testing? Bad water quality will make your fish prone to ich. Are the fish new to you? Ich is a pretty common problem in new fish because of the stress of transport and being in lots of other tanks before they come to you.

If you don't know about it read about the life cycle of ich as it will help you understand how to get rid of them. I'd do some big water changes, hoovering the bottom of your aquarium well, that will help with water quality and remove some of the free swimming ich before they get to the fish. Salt has worked well for me in the past.