All our fish were dead this morning - one Orange Oranda 12months old was happy as larry, one lemon Comet 9months old happy again as happy as larry and a calico coloured Ryukin 6weeks old[we've had it two] could he have brought a disease even though he looked fine??? the other two have been no trouble in the time we've had them - lost two red cap Orandas [the comet was a replacement for the first and the ryukin for the second] previously but assumed these were a little fragile. When we bought the Ryukin we also got two apple snails - could this be the problem. Tank is 40l and has a filter and two air stones Water changed two weeks ago this Sunday. Any advice welcome.