could easily be any of the above. just wanted to comment on the phosphates. not saying they are what caused any deaths, but if you want to reduce phosphates, use some RO water for your water changes. half RO and half tapwater is a good combo for your new water if you wish to reduce phosphate.
regardless, phosphates are usually not the cause of fish deaths. usually you will just get an algae bloom when phosphates are high. more algae will eat more phosphate and it can level itself out after awhile.
your friendly neighbourhood arowanaman!