Hi,
Wow, nearly 500 watts - that's like a Vegas nightclub. I'm stumped on what species of algae this is but it really doesn't matter, the cause is fundamentally the same as most; too much light coupled with what would be almost certainly not enough nutrition, coupled with ammonia leaching from the dying leaves. I can almost guarantee that you'll not get a handle on this until you sharply reduce the lighting. It seems you've already taken Hoppy's advice and dropped the intensity by about 200 watts but in the short term I would slow things down even more. I know this might sound draconian but you would be amazed at how much easier your life would be if you cut it down to under 200 watts. I know, I know, it's difficult to resist the temptation to throw megawattage at the tank and to justify it because of it's large volume but you need for the plants to develop enough to compete properly. The Cypts are slow growing so they will require a lot of time. It would help to get another 100 stems of any cheap hygrophilia. I see a lot of open space in the photos so you need to stuff the tank to the gills. Remember, this is just short term, you only need them to help you stabilize the tank and after a few months you can start to replace them with the plants that you really like.

Assuming that you do a blackout, RUTHLESSLY trim off infested leaves and then stock with more plants you should study the following thread until you can recite it by heart:
The Estimative Index of Dosing, or No Need for Test Kits

Dosing schemes can be found here:
EI light: for those less techy folks

I would use the 100-125 gallon scheme listed near the bottom. For the next few weeks I would also do more frequent water changes, say 50% 2 to 3 times a week dosing immediately after the change.

CO2, there is no way I'll believe there is exactly 21 ppm, the measurements just aren't that accurate. If you pump enough gas in the tank to keep the drop checker the color of the interior of a lime you'll be OK. It should be that color by the time the lights go on. You can turn it off after lights out if you wish.

You may also consider some algae eating fish. Fifteen or so Otocinclus would do you a world of good for that size tank. They are peaceful, tireless and you don't really have to feed them for the duration.

If you do the above you won't have to test for all those parameters, and I doubt that the readings are very accurate anyway.

After a while, when the weight of the plants increase and the tank stabilizes you can start to prune and to slowly increase the lighting to your favorite Vegas levels, keeping in mind the possibility that the dosing and CO2 will have to increase as well.

Cheers,