This is not new tank syndrome unless that means placing a bio load in a tank before it is ready. Your tank may have cycled but that doesn't mean you can start loading fish into it. It takes time for a tank to build up enough bacteria to handle fish. Two and a half months is sometimes not enough to even finish the cycle and definitly not enough time to have that many fish thrown in. And yes, it happens to many people as testified here. But it is no great mystery. They did the same thing you did. The reason is impatience. It just takes time to do it right. So do as was suggested and get the fish out and do a lot of reading.