Just, be sure to call before returning them to find out if it might be a problem. Stocking or adding fish to an aquarium should be done over time and gradually. The environment has to adjust itself to the new inhabitants and this requires time. Patience is one of the necessities of the hobby and it will pay off in the long run. Count your fish.
Dead fish can decay in just days, creating a spike in ammonia and nitrite .
Thus leading you to assumptions of a failing filtration system or other possible reasons.