I never make assumptions on things like this where the tank owner is not involved in the conversation. For all we know he has a 500 gallon sump tank and the nitrate never gets above 5ppm. I know that's unlikely, but it's not impossible. Another "what if" is mabey this is a holding tank and the fish are just there for a day or two untill s/he decides which of the other 20 tanks s/he has they will be living in.