When i woke up this morning i looked at my tank just like a do everymorning and saw that one of my angels (4 years old) was dead. The angel had not floated to the top like normal it was on the bottom of the tank but clearly dead. All the other fish seem fine. I tested the tank and there was no sign of any toxins. The temperature was at 80 the same as it has been since i first got the angels. any help would be great i dont want to lose anymore fish.