I use a range of test kits - JBL like you for KH and GH. I've got an electronic pH meter. I use lab grade kits from Merck for Nitrates and Phosphate and an AZOO Fe test kit. Although the only ones I really use regularly are the KH, GH and the Nitrates. The all seem accurate enough +/- 1 degree for the KH and GH when I've made up some standard solutions at work.
As for water parameters - I'm no expert at all at this and have only had success at growing plants over the last six months or so after years of algae factories. I got better results by getting my nitrates down, upping the CO2 to around 30ppm, and dosing ferts. My main tank is only medium light 2.3 wpg but I add CSM+B for iron, MgSO4 for magnesium, K2SO4 for potassium or KH2PO4 for both potassium and phosphates when needed (I get these from Greg Watson and Aqua Essentials, both are really helpful). For the CO2 level, try taking out a sample of water and leave it for a day and test the pH. Aim for a pH of 1 lower than that, alternative lots of people suggest turning the CO2 up (very) slowly until you see the fish begin to look stressed and then back off 0.1 or 0.2 in the pH. Don't always believe the CO2 charts as they are only valid if the only buffer is carbonate.
My parameters at the moment are about
KH 8 d
GH 11 d
K about 20 ppm
PO4 3-4 ppm
Fe 0.1 ppm
Mg about 10ppm
If you want to reduce the phosphate in the tap water, try some JBL phosEX ultra. Put it in a little internal filter and leave running in the new water in a container over night before you do the water change. Although I do my water changes with HMA filtered water (filter from from Devotedly Discus) and leave the phosphate in.
Hope some of that helps?