I love when you realise you know answers to things
Update: I visited the LFS today. I am in the process of trying to decide which is the best LFS from 2 that are local... one is an aquatics shop who are well respected for freshwater, but not sure on marine, they've only just set up the marine section.
... the other, have a fab aquatic department with more marine than freshwater - some superb fish, 10ft display tank etc etc, but are new (open for a little over a year) and also sell reptiles, small pets and parrots...
I want to have one place I can trust I suppose, though anything I am told I will double check online anyways!! So, we visited both today, the first one, the petshop was really busy, we looked but couldn't get any attention from staff due to them being with other people... so we left and went to the aquatics shop. There we chatted to a guy who seemed super knowledgable, definitely full of info, showed us pics of his 2 tanks, seemed to knows his stuff. I mentioned that we had only just bought the RO unit and hadn't got any further, and he said an RO unit is a waste of time... he said our local water is (if I remember correctly) DKH - 12, Nitrates - around 5 and PH around 8. He said an RO takes
too much stuff out of the water and I will need to add things back?? He said that he uses a water filter to remove some stuff... any idea what kind of water filter he could be talking about??? and that he has never had a problem... any thoughts?
So, we looked at the livestock, all looked great, no dead fish or anything... and I spotted something he told me was a black sea urchin.... I told him we planned on having a pair of clowns, a royal gramma, a firefish goby and a bengaii cardinal and he said the urchin would be fine?? Is he right? More importantly... do they have any special care requirements??
As always, any info or advice would be very much appreciated!!
I'm confused 
A good salt brand (most of the major brands available at your specialised SW dealer) have everything you need to replace in the salt mix. RO, standing for reverse osmosis, removes 99%* of impurities from your water (* this will also depend on the type and quality of unit you buy, or your LFS uses). This includes fluoride, Chlorine, Chloramine, Iron, Silica etc etc. That way your left with what is essentially pure water. Adding salt to this, and doing regular water changes will replace any trace element needed by corals or inverts. Many people add ALOT of products to their tank, without even checking if that specific element is being depleted. Dont add what you dont test for, because frequent water changes will replace the majority of the trace elements needed by any animal in your tank.
