Saltwater is an ancient soup of mineral's that fish, and us, are dependent up. When sodium starts to accumulate out of your water, at the cracks of your sump or some other suitable point in your tank, those minerals disappear as well at differing rates. Your fish and invert's start to miss them in varying ways. This depends on the creature and what it's used to in terms of water quality, of course. The effects can be as simple as lack of appetite to loss of color to death, and everything else in between.
When I tried saltwater, I ending up buying anything I could afford to stabilize things. None of it worked, until I found a major issue. My wet/dry was situated in such a way that the back of it was out of sight in the cabinet. I had to clean it because the spray bar had stopped. I immediately noticed that there was a large crusty lump of salt on the back of the sump and bottom of the cabinet. I don't know for sure how this happened and got so bad, but I do know that I had wasted k's of $ and scores of animals. I did what I could to rehome what was still alive and gave up on salt forever.
Bear in mind, this was almost 30-years ago, so hopefully salt-tank tech is much better and easier to understand now. If you can, please post a pick. I'd really like to see your work and stock. If you want to see what I mean by "soup", take a look at the ingredients that come in a bag of top-quality
aquarium salt.
BTW, I currently use a Jebao\Jebod SCP-180 wavemaker (the cheaper version of a Gyre). Even though I do fresh only now, it's improved my water in some very positive ways.
