What is the most common problem/disease found in SW tanks?

[illuminum]

I plan on starting one up soon and am trying to see what I'm likely to come across.

 

[loganj]

For the fish, probably ick. If you follow proper procedures though, you can minimize the chances of any of your livestock getting sick. I would go with a fishless cycle and quarantine all livestock...that's as close to a guarantee as you can get.