I had a 55 fresh and a 20 tall guppy tank. Went salt and never looked back. Now I have a 55 reef, 50 reef, 2X30 reefs, a 2.5 mantis tank, and just broke down my 55 fresh to become another reef. I have a 180 sitting waiting for me to have time.
By the way, noone mentioned this, so I feel it is necessary. I don't know how much you newbies have read, but it is tough(but SO worth it). If you are just now setting up the tanks, you are not ready for coral. You have to put live sand, live rock, and water in a tank and watch it empty for a couple months. There is a cycling period where dead bacteria dies off and goes through(usually) an ammonia, nitrite and finally nitrate cycle. Once this is done, then it is time to start worrying about coral. If you put it right in on new rock that has not had time to diversify biologically, you are dooming the coral to a slow and painful death. Let the tank settle before you start adding stuff and you will be much happier in the long run. Just a word of advice...
This hobby is a slow one. There is a saying, "Nothing good happens to a reef overnight." You will need patience, time, effort, and some deductive reasoning skills, but the end result makes every second and every dollar you put in all worth it. After all, you are sustaining a whole community of organisms, not like a dog or cat. With a dog, you know when he's happy, you know when he needs to go out. With a community, one small thing might not be happy while everything else is. This one thing can make a lot of other things just as unhappy as he is. You will not regret coming to the reef.