The third graphic on this thread shows the fixture you have. Tons of light. Even the 2 bulb version needs to be hung about 25" above the substrate for a reasonable amount of PAR. If you wanted to hang your 4 bulb 30" above the substrate then that might reduce the PAR at the substrate level to a level you could handle.
I see no mention of fertilizers? With a bright CO2 enriched tank you will need to add a complete range. Unless you can target specific nutrients that are low and dose just those it is far easier to overdose everything and make regular large water changes which is the basis of Estimative Index. See the sticky in the Fertilizer forum for how tos. Find a vendor, several present on this forum, buy them and dose according to the chart on the sticky. Make that large water change and done.
I like doing water testing but what my tests come up with is more how the tank is trending rather than the value as hobbyist tests aren't very accurate. Same with the drop checker. I think they are very attractive gadgets and if it is still blue at the end of the day something is wrong with the system, maybe the tank is empty or there is a kink or leak in the line, but in the end it is how your critters are doing that counts. Bump the CO2 up a notch every couple of days and watch very closely. My platies start hanging at the surface and the fish get generally cranky if CO2 is a bit too high. If I see a black neon tetra bump the surface then that is trouble. Maintain a good ripple on the water's surface to get lots of oxygen into the water. If there is lots of oxygen then the critters can handle more CO2.
I really like my tank better when I do larger water changes. Figure out a way to make them as painless as possible so you can do them more often. It gets odd bits of debris out that algae might use as a substrate, removes organics in the water and just brightens things up. I happen to use a python that is long enough to reach the garden so the water isn't wasted then remove the aerator on the kitchen faucet so my hose end adapter can go on to refill the tank. I just dose the dechlor into the tank. Last summer the tank missed a couple water changes and I came home to loads of BBA.
I have Staurogyne repens. Pretty plant that actually looks a bit like the hygro you have minus the pink tops. It likes to melt which in this case means shed leaves. Then the bare stems grow new leaves until the plant decides something is missing when it will shed again. As long as the stems are green it hasn't given up on you yet. You trim by snapping or cutting the too long stems and they can be replanted to thicken the stand. The stems are fragile as well, take care planting them. You must use fertilizers with it! It loves CO2 but doesn't seem to need massive amounts of light. Suspect it would appreciate root tabs as the root system is massive.