First, general warning about all of these tests - Some of these tests (notably Nitrate) are notoriously inaccurate. Most of these tests are made to keep costs down and be good enough for estimation and/or relative comparisons needed for troubleshooting. I would not trust any of them to provide data that is accurate enough for any kind of scientific experimentation.
That said, if measurement is your goal:
- There's pretty readily available API phosphate and copper tests. I have the phosphate kit, it is pretty coarse in terms of measurement, but works well enough for me to get a rough idea if my P levels are somewhere near where I want them or not. Its scale is 0, 0.25, 0.5, 1, 2, 5, and 10ppm, but 0.25 is hard to tell from 0.
-There aren't many potassium kits out there, mostly because few people care about what number this parameter is. Too high a level doesn't cause problems, unless you get absurd... To low causes easy to spot plant deficiencies. So, most planted folks that need potassium just overdose it a bit and let it go. Most non-planted folks don't care what the level is. Same goes for Mg.
- Seachem and Nutrafin both make an iron test kit. I've not tried either. I don't think many use this test, except those doing experiments like you are or those with high-iron water supplies causing problematic levels. Again, if you need Fe, it is pretty easy to just react to symptoms.
If growing plants aquariums is your goal:
Spend some time over at http://deficiencyfinder.com/
and start learning what the various defficiencies look like. This will save you a lot of money over trying to buy dozens of test kits at $10 a pop.