I do not want to go off topic. However copper in the closed marine aquarium is a great concern. Of greater concern are high levels of metals not easily tested by marine aquarium hobbyists. There are other factors that can attribute to inaccurate copper test results.
Copper levels in Aqua Craft Products marine salts is similar to levels found in natural sea water.
For accurate testing of low levels of copper, try the following:
Add 1/2 the amount of aquarium water in the test vile with your copper test kit. Use the same amount of test reagents. This will offer a close approximation of what copper amount is found at 50% of standard testable level.
This approach can be used with PO4 test kits for learning low levels or lower than the kits standard testable levels.
It is essential to use a copper test kit that will provide accurate measurements of the type of copper you add in your aquarium. E.g. unchelated copper must be tested with a kit that is specifically designed to test for this copper form. Chelated copper must be tested with a test kit that is specifically made to test chelated or bonded copper.
What "daily chemical dosing" gives you concerns about adding Cu? Have you tested the freshly mixed marine salt you currently use for Cu?
If you use RO, RO/DI make up water and you test freshly mixed marine salts and you get concerning test results for ammonia, copper or silicate.... Step up to better initial water quality. E.g. Pay a little more for better initial water quality (better quality marine salts) for your captive marine pets.
Periodically low purity sulfate can have high levels of antimony. Antimony (Sb w/atomic number of 51) is sometimes erroneously identified as led (Pb). Sb and other metals can interfere with some aquarium grade copper test kit results. The largest applications for metallic antimony are as alloying material for lead and tin and for lead antimony plates in
lead-acid batteries.
I.e. We were approached by a sulfate producer for possible use in Aqua Craft Products marine salts. Upon investigation we learned that this suppliers sulfate was derived from recycled auto batteries. We asked for a 2 Kg sample. An independent lab. utilizing atomic absorption and spectrographic analysis confirmed high levels of Sb (2000+x higher than NSW).
There are at least two European producers of marine salts where independent laboratory testing confirms significantly higher levels of: aluminum (Al), lead (Pb), nickel (Ni) vs. levels found in NSW. These two EU producers of marine salts currently produce no less than 5 different brands of marine salts. One with 3 brands with their name. One with one brand with their name. One of these producers is private labeling under their clients brand. All of these brands are available worldwide.
What is interesting is that these EU producers have openly indicated (in print adverts. & on their respective packages) their brands are made with pharmaceutical grade ingredients and/or are identical to sea water. If these brands do employ pharmaceutical raw materials, one would wonder why very high levels of aluminum, lead, nickle are found in their finished products? Do they add these metels at these amounts? If so, these brands are not "identical" to sea water. If they add these metals at these levels, why? One could speculate that these metals might be present as impurities found in non-pharmaceutical ingredients? One might also wonder how these brands could be price competitive if 100% pharmaceutical grade ingredients were used (cost might prove to be prohibitive).
There is at least one US made marine salt that claims to be made with pharmaceutical grade ingredients. A few emails and phone calls confirms that this brand uses one or more forms of Mg that are made in China and are technical grade. The Mg is purchased in the USA and initially is described as USP. However, when samples of this very same material was sent to us, it was labled as "technical grade". When I called the supplier it was revealed that this is imported material from China and is in fact technical grade.
Steve, what size is your marine aquarium and how much water do you mix at one time?
MDP
after reading many articles Seachemcopisorb may be a potential solution to the problem. I want to hear from those that had issues and those that have experience using this product. 
