How many gallons is your tank? It is probably the rainbow shark, which should not be kept with smaller species.
Rather than jumping to conclusions (I've had rainbow sharks with neons and had no issues in the past) we should probably ask more questions.
Did you cycle the tank? Do you have a test kit to give readings for ammonia, nitrite, nitrate? Did the fish look healthy in store? How long generally did you have them before they started dieing? Did you notice anything odd about the dead fish (markings, red patches etc)? Any idea what type of pleco you have, if not, any way of posting a pic of it and the eel so we can help identify?
Answers to all but the last question should hopefully be able to clear up the cause.