I have a 250 gallon saltwater tank set up with about 10 fish (large to small). This tank used to be an aggressive tank turned to a reef tank. All of these fish have been in the tank for more than 6 months. Nitrates are higher than they should be but has not been an issue before. Now when I add new fish they survive for weeks - they eat fine and no one visibly picks on them. All of a sudden the new fish will stop eating and be dead within a day with no visible stress spots or behavior . Anyone have any thoughts? All of the old fish have no issues.