Tank looks very nice. None of those additives should drop ORP the way some things do (such as vitamin C). I'm not sure why you are seeing what you are seeing.
I would not expect GHL to be using a different standard, but there are two ways to refer to ORP. One is vs the standard hydrogen electrode (not usually used by reefers), and one is vs the silver/silver chloride electrode. I could not find GHL stating what they use.
I discuss that issue below:
ORP and the Reef Aquarium - Reefkeeping.com
ORP Electrodes
Since ORP is a measure of the electron "pulling and pushing" from the solution, it makes sense that ORP would be measured as an electrical signal. In other words, the chemicals themselves pull and push the electrons to and from a suitable probe, and the resulting voltage is a direct measure of the redox properties of the solution. ORP can be measured in other ways, such as with redox sensitive dyes, but that is rarely done by aquarists.
The electrode that actually does the ORP sensing is usually an "inert" metal, such as platinum or gold. However, one cannot simply put a single electrode into a solution and expect to get anything useful because the voltage needs to be compared to something else. That is, voltage is always the electrical potential difference between two different points, not an absolute measure at a single point. So one needs a reference electrode that provides a constant "ground" with which to compare the electrical potential in solution.
One such reference electrode can be provided by the standard hydrogen electrode. The standard hydrogen electrode involves the following reaction:
2H+ + 2e- <--> H2
The standard hydrogen electrode is a probe immersed into a solution with a fixed concentration (activity) of H+ and H2. This reference electrode is arbitrarily defined as having a zero voltage. To make a measurement with a platinum redox electrode, one then measures the voltage difference between the platinum redox electrode put into your solution of choice, and this reference electrode put into its reference solution (plus an electrical connection between the two, usually provided by a salt bridge). The value measured in this fashion is often called EH.
Unfortunately, the standard hydrogen electrode is cumbersome to use, and only specialized labs typically use them. Luckily, much simpler reference electrodes have been developed that are very easy to use. The one that is typically used for measuring ORP is the silver/silver chloride (Ag/AgCl) electrode. These electrodes are typically included in ORP electrodes, even if the manufacturer does not specifically say so. Consequently, all ORP readings taken by aquarists (and all values quoted in this article unless otherwise stated) are using this reference electrode.
The
Ag/AgCl electrode works as follows. Inside of such an electrode is a silver wire coated with silver chloride and surrounded by a solution saturated with potassium chloride. The reaction setting the potential for this reference electrode is:
AgCl(s) + e- <--> Ag(s) + Cl-
The potential for this reaction only depends on the concentration of chloride in the internal filling solution. Using a saturated potassium chloride (KCl) solution keeps the chloride concentration steady (at a given temperature), making this a good choice as a reference electrode. One then only needs a tiny electrical connection to the solution being measured to complete the circuit, and allow measurement of ORP using a platinum electrode.
One difference between the Ag/AgCl electrode and the standard hydrogen electrode is that they do not have the same potential voltage. If they did, and the potential difference between them was measured, there would be no voltage difference. However, it turns out that there is a voltage difference of about 199 mv at 25°C. Consequently, if one wants to interpret ORP in terms of the EH, one has to add 199 mv to the ORP reading to get EH.
