I would have the LFS use a TDS meter in front of you, of course ask in a way without being rude. Or just buy an RO/DI unit, always good sales going on and usually have a TDS built in. That way you know what you are getting for water. And it's always on hand if needed. Nassarius snails won't do much for you on your rocks, get some Ceriths, turbos and some other grazers. Hermits will help out too. Your Melenarus will pick them off slowly, but if you get a large amount, ( 100 snails, 50 hermit)it'll give them time to help before they get thinned out. Maybe look at some conchs for your sand bed. They really do wonders, I've got 6 in mine and they are always working.
Wet / dry skimming all depends on how you tune your skimmer.(air intake) I like my skimmate to end up the color of a dark iced tea. If your skimmate is ending up lighter it means it is wetter, more water than waste. Sometimes just a small adjustment can make a huge difference.
How are you feeding your nori? Tied to a rock, drop it and forget it? I was doing that to, and found that there was a ton of waste, and I was going through bags quickly. Innovative Marine has a cool new grazer that traps the nori sheet in a grid type holder that looks very promising. I've been using the New Era Marine Grazer rings. It's a compact nori ring that attaches to a suction cup. I'll put it in for a few hours every other day. When I take it out, I'll let what is not eaten dry for a day, then put it back in the next day. Just got to remember to not leave it in all day or it will break up, fall and rot in some rocks. Which is what I found was happening with the sheets of nori.
The RW15 sound like a good powerhead, but you only have flow from one direction. I mentioned how much flow I have in my tank, this is coming from 6 different positions, hitting the rock work from the front, sides, behind and above. I have two tunze 6105s behind my scape and down low pushing through the rocks and up for surface agitation, two tunze 6090s higher up just above the scape, all four on a controller. My XP150 is pushing from right to left on a 2 minute cycle of forward and reverse and then my return has a flow accelerator with a rotating outlet. Nothing is at 100%, but there is not a visible piece of rock that does not have good flow. I have gotten to the point where I can turkey Baister blast the rocks once a week, and there is not much build up at all.
So I would say get a large cuc and a RO/DI unit, then start shopping for some powerheads. Your tangs will love the flow too. Get to turkey Baister blasting and change your socks two or three times a week to start. Run some Rowaphos in a reactor if you can, or in a bag. But the MR-1 reactors aren't to $$$, and worth having. Maybe start looking at bigger sump options also. I know it sound like $$$$$ , but as your tank matures, corals grow, fish increase, coral tastes get more expensive, you'll be thankful you spent the money now to make the bones of your system bigger, stronger and more available to upgrades as you need them. It's better to have the room to add, then trying to play catch up
Looking forward to seeing some pics