Think it could be due to a pH swing. A tank at warmer temperatures, has less oxygen carrying capacity. Coupled with higher metabolic rates, there will be higher CO2 content in your water. If your kH is low, chances are that your pH might have fluctuated quite a little. Stressing the livestock. The first to go would be the shrimps..then maybe when the shrimps became 'belachan' then the fish became affected. Just my guess...