Alarm won't stop

I am with a aquarium retail store in Georgia. A customer purchased your ATO and after a few weeks returned it to the store because the alarm would not stop beeping and the pump overfilled his sump. I refunded his money, then set it up and had the same result. I know ATO's can be tricky but after seeing the other comments it seems that the Coralvue ATO is not a reliable product.