Gunthr,
its all about moderation and adaptability.
Some folks jump in and put every cool looking coral and fish in a tank and Ill tell you what it will die off or get very ugly quick.
you can get by without using RO/DI but you have to know the specifics of your water. In Fort Walton Beach, where I lived prior to Japan, I had no use for the RO/DI....my corals did fine as did the fish....partly due to the fact I dove off the Gulf with friends who owned a reef store and harvested liverock with them, and partly to the fact I didnt have a problem going to the beach (5 miles away), and hauling those damn plastic jugs up.
I just let them sit for 48-72 hours before adding my top off water. You have to be careful, Im sure you know this as your pic shows you have a beautiful tank, that if you to replace with seawater or mixed...CHECK THE SALINITY. I would dose with just regular Winn Dixie Distilled water when I didnt want to go to the beach to get salt water.
BTW it also depends on how a person wants to set up their tank.
In my FW I have CO2, 2 EHIM cannisters, PH meter, and a pump in the tank to circulate things. MY lighting in the 120 is an electronic ballast (btw same i used in my SW tanks!) and it kicks arse.
I keep things on the simple....my other tanks are all run on substrate on the cheap like kitty litter and playsand and absolutely rock and roll!
Again its about knowledge and starting slowly.
Where are you in S FLA?
Regards,
mike