I have a 250 gallon tank. once my tank cycles, i plan on doing a water change between 1 and 2 weeks. The place I bought the tank from sells pre-made salt water. He said I didnt need to worry about checking the salinity as long as I did my cleanings and water changes regularly. Do I need a hydrometer? I am sure I will get both yes and no answers. Believe me I really want to know. I love all animals and dont want my fish dying because my salt is too low in my tank.