Water chemistry is water chemistry. The relationship between calcium, magnesium, alkalinity, and pH does not change.
This topic is so complicated i'm not sure where to begin. So much depends on stocking levels and type of filtration. Systems which rely heavily on biological filtration to process waste will have a greater degree of difficulty in maintaining these levels. Systems which are set up correctly, in my humble opinion, using a deep sand bed, live rock, and a protein skimmer, will have less difficulty in achieving stable levels. The removal of organic acids by the protein skimmer, as opposed to the biological processing of organics by a biofilter, goes a long way to allow for stable alkalinity levels.
That being said, to maintain a stable pH and properly balanced ions, you will have to test for alkalinity and calcium. I will point out that it uncommon to have both levels high or both low... it is more common to have one level elevated and the other at the midpoint or slightly low end of the range. This is why we try to keep these levels within a given range, calcium between 360ppm and 500ppm, and alkalinity between 8 and 12 dkh. The addition of calcium, even in a fish only system, should be utilized. My chemisty is not strong enough to explain the cause & effect of this relationship, but i know Steve can elaborate or provide some nice links to articles on the subject.
For any system, i would suggest testing alkalinity and calcium weekly, adding the necessary buffer and calcium addition as necessary to stay within the given range. I would personally add calcium daily and buffer twice weekly. The size of the dose will vary based on your systems test results.
As to magnesium, many hobbyists advocate testing. I have personally never tested magnesium and have no plans to. However, i do biweekly water changes and believe this should be sufficient to maintain a proper level. If the time ever comes where my calcium levels are at the proper level and I am still unable to buffer to a correct alkalinity level, then I will have to reconsider my position on magnesium supplement and testing.
Iodide testing in a fish only system would not be needed. In fact, many in the hobby today do not add iodine or iodide to their systems at all and are more concerned with OVERdosing iodide than not having enough in their systems. I do not personally add iodine to my reef.
As to phosphate, I also do not see the need for testing. Interpreting the results would be difficult. A zero reading could simply mean that phosphate is being utilized by algae faster than it is being introduced into the system, but certainly does not mean phosphate is never present. I use my eyes as my favorite test kit, carefully observing for unwanted algae growth. I take steps to reduce phosphate buildup, such as a daily cleaning of all filter pads and utilizing proper water movement to prevent detritus from settling.
RO water would be helpful in any sytem, but many hobbyists do not use RO in fish only systems. Myself included.