The school I work at has a planted community tank. There had been 4 albino corydoras, 4 cardinal tetras, some amano shrimp, and 2 nerite snails living in the tank for just over a year with no problems. Last Wednesday we added a few more amano shrimp to help with algae control. Last Friday when I left for the weekend, everything was fine. On Monday, all the fish were dead, but the shrimp and snails are all fine. I checked the water parameters immediately upon discovering the dead fish, and nothing seemed obviously wrong. The water temp was 28 C. Before we get any new fish, I want to make sure that the tank is ok. Right now my only theory is that the water got too hot over the weekend. However, the fish survived last summer without any issues. Does anyone have any ideas as to potential causes?