I have a Biocube 16 in my classroom. I've had this tank for approx. 4 weeks. On Tuesday (5/25) I cleaned the substrate and replenished the water I suctioned out with fresh saltwater. The only difference is instead of using Prime I used API Tap Water Conditioner. Once complete the tank ran for approx. 4-5 hours before I checked the water for salt, nitrite, nitrate, and PH. Each checked out fine. On Wednesday (5/26) I walked into my classroom and went to check on my Bicolored Angel Fish, Chocolate Chip Star Fish, and Red Fire Shrimp were all dead! The Biocube was running perfectly. I again checked all of the water parameters and they were fine. The water did have a metallic smell to it. Does anyone have any clue what would cause everything to die between 5pm - 7am?