I got completely lost in this thread :shock: So I'm just going to start over, even at the risk of having missed a bit of info LOL
Ok, so first things first, how do you acclimate your fish? Were they eating at the lfs, then were they eating at your new tank? It's likely you just got bad fish from the LFS (Petco is a hit or miss -- bifferwine had great fish from Petco, but my local Petco has sickly fish)
I'm not so worried about fluctuating Nitrates especially under 10ppm. Hell, my tank's a fowlr and my nitrates haven't always been great lol
I missed if you checked for phosphates. I must agree that the hair algae is coming from somewhere. Are you using tap or rodi for water changes and top offs?
I also agree that you should be topping off daily. I have a 125g plus a 60g sump. I installed an ATO to make sure my salinity doesn't swing too much from evaporation. I can't imagine my salinity after 3 days of not topping off. It's possible your salinity swing could have stressed out your fish.
+1 Smitty, maybe you have a predator that you don't know about....hear any snapping/popping?
Since my tank is fowlr, I can get away with no water changes for a while (not a good idea, but I get lazy sometimes...I've gone months without a water change...I don't recommend it, but it happens). But since you have a reef tank, maybe up your water changes, too but if your levels are really in check like you say, then you should be fine with what your doing.
Just keep an open mind about some of the advice, though. Think about this: If your maintenance procedure is really working great, how come you're losing so many fish? Maybe it's time to at least just consider what some people are advising you here. Then again, you literally could just be getting bad stock from the LFS.