i have a 42 gallon tank. I am starting brand new in saltwater and I am only going to do fish. I put in a little to much salt and I am replacing it with fresh water to balance out. I am not getting a good reading on my hydrometer. I dropped it down 6 in down and let it fill up I take it out and sit it on a flat surface and it seemd like the hydrometer needle just sits at the top and then some times it moves down am I doing something wrong in reading it.