I'd say that more people add things to their tanks that are supposed to REMOVE things

if that makes any sense.. lol. I think it's fair to say that currently people are closely watching their calcium, alkalinity, magnesium, pH, phosphate and dissolved organics.
Calcium reactors are a popular way to constantly replenish calcium that is used up by coralline algaes, invertibrates, SPS and LPS, but calcium chloride and calcium hydroxide (Dow Flake & Slaked Lime) are also widely used as a regular supplement.
Alkalinity can also be controlled with a calcium reactor, but those who preferr manually adding supplements usually use Sodium Bicarbonate (baking soda) or commercially available buffers. Alkalinity helps to stabalize pH, which in turn effects solubility of other nutrients such as calcium.
Magnesium is also linked to solubility of calcium, but more imporantly should be looked at because it is the third most abundant ion in natural sea water, but salt mixes are often defficient in Mg. Common additives are Mag Flake, magnesium sulfate (epsom salts) and magnesium media that can be put into calcium reactors.
Many people are concerned with certain metal ions, phosphates, and dissolved organics because they can inhibit coral growth and the general health of a tank. Ion exchange products such as Rowaphos, Phosban etc. are common for removing phosphate, and I believe some of them remove silicates and other ions as well. Ion exchange media is not commonly used to remove metal ions directly from the tank. Activated carbon and protein skimmers are commonly used to remove dissolved organic compounds and proteins to maintain clear water and deter algae growth.
I digress.. The most common additive is undoubtedly water changse. Water changes are the least expensive and most effective of all supplements you can buy (in my opinion). Many people in this club feed their tanks with phytoplankton products such as Phyto-Feast. Also, you mensioned copper; copper is the Reaper to a reef and shouldn't be a problem unless one contaminates their reef with copper treatments.
Good luck. Just ask if I missed your question.