Isotonic and dilute are terms used to compared the surrounding environment. Marine fish want to keep all the water they can b/c the environment is so salty so the best they can do is have urine be isotonic (the same tonicity as the surrounding environment). While the fresh water fish want to keep as much salt in their body's as they can b/c their envoirnment has so little salt, they have dilute urine compared to the surrounding envoirnment.
