Yes. In a "freak" accident we lost over 300 fish in a system in a matter of hours. We did discover that chemical poisoning took place. Foul play and a series of unfortunate events. If you know the basics of anatomy and pathology, you can get by and figure out most common aquaria problems. Just keep in mind that most systems are dependant on each other and a problem with one will cause other problems.
For example, in the death of 300 fish, chemicals attacked the gills first, causing them to turn white, and eventually mush as the cells broke down. The kidney also turned to mush and turned black. After these two vital organs failed, death was fast and all other organs were in relatively good shape. Water chemistry and other tests showed nothing wrong. It was determined that bleach was poured into the tank, and with bleach dissipating quickly in water you would not expect to find a trace by the time that test was performed.(more evidence but that is fine for the example).