I find that a little hard to belive, but assuming you are not trolling, when fish urinate where do you think that goes? Fish wastes do not generally disappear into thin air.
Normally I would say change 10% or a gallon or two a week, but I tihnk you might want to do a bit mroe than that. Do you buy or make your own salt water. I would be tempted to take 5 gallons fmor the tank, and then put 5 gallons of new water in , one gallon every hour.