Get you a "Salinity Refractometer" a little more money but much more acurate and easy to read. most swing arm hydrometers will give an unaccurate reading when little air bubbles cling to the needle.
Uh.. i bought mine at a dedicated Local fish shop. they are located in charlotte NC by the name of Fintastic... maybe you can call them so they can give you a price quote. you could also buy them online.
lol! yeah the page i looked at at had it for all types of animals!
just make sure that the one you get has the mesurments for fish. i dont kno if the #'s for a salt water and dog pee are the same