My dad recently said he wanted to try salt water. I have a 15 gallon downstairs that I could use if I needed to use it for saltwater, but is that to small or is that a good starter size. What could I stock in a 15 if I did use it or if I went up what could I put in maybe a 20 or 25 gallon tank? Would I need a skimmer for the tank or would it be fine without it? I just need help with saltwater because i've never done it.