Water changes are necessary. Water changes reduce accumulated waste ions. Water changes replenish elements used by the biological filter as well as metabolized by our captive marine pets. A decent marine salt will "recharge" the closed system with high quality fresh salt water that is ionically balanced.
There are several brands of marine salts that vary in copper content from batch to batch. Levels of Cu in these brands can range from: 0.15 ppm to 0.24 ppm. (0.15 ppm. of Cu is the treatment dose required to cure ich. 0.15 ppm. or more of Cu can kill sensitive invertebrates.) These same brands, from time to time can test for ammonia at 0.25 ppm or more.
There has been enough discussion on various marine aquarium web sites that detail the brand names of these offending marine salts in the last few years. As one might think, these brands should be avoided.
As speculated by others, using various chemical filters in an effort to lessen the water change routine can actually cause negative chemical shifts within the marine aquarium. E.g. Some chemical filter(s) can bind heavy metals. If pH drops, accumulated copper can be released into the marine aquarium.
The KEY is to use a decent marine salt. And perform regular water changes.
A decent marine salt will not add: copper, other heavy metals, ammonia, silicates etc.
I.e. We have one commercial customer with dozens of aquariums totaling an 8000 gallon system. They change 150 gallons of salt water each week. They employ a variety of chemical filter mediums. At the same time they are adding various supplements to replenish ions removed by the chemical filter mediums.
After a quick look at how this customer maintains their aquariums, it was evident they are "running on the fly". E.g. No clue as to the ionic content of all 78 elements found in NSW. They simply test for Ca, Mg, Alk, No2, No3, NH4. When nitrate increases, they added more "nitrate" resin. If water turns yellow, they added more carbon. Then added a bevy of liquid and powder supplements.
For some months their fish and corals looked very good. Then the inevitable occurred. Their fish continued to look good. (They keep fish for an average of 1-3 weeks prior to sale then restocking). However costly corals that were for sale / on display for over 6-8 weeks began to appear in poor health. Large or show pieces of very costly corals that were kept for over 2 months began to die off.
The owner refused to acknowledge the possibility that various chemical filters were actually releasing accumulated metals. This shop uses and promotes the approach of chemical filtration and supplementation vs. simple water changes.
When testing for copper it is essential to use a copper test kit that will not give false positive test results if other attributing factors are in play.
When using a marine salt it is essential to use a brand that does not contain elements in dramatically disproportionate amounts (vs. NSW) that can give false positive test results with some brands of copper test kits.
