- KH 4 is PERFECTLY FINE for Ammannia gracilis. There are other Ammannia that might prefer KH of 2, but gracilis does not care. But cutting your tap with RO 50:50 is fine and probably a good thing overall.
- Ignore the advice you were give about micronutrients not being absorbed at KH 4 and whatever pH that was. That's bull sh*t.
- Calcium of 30 ppm and Mg of 10 ppm are fine. I assume you are adding GH Booster (Calcium sulfate and Magnesium sulfate) after each water change. Assume your tap was does not have much of either.
- 50% water changes are nice. 75% is better. Once a week is nice. Twice a week is nicer. At least until you have issues under control.
- EI is fine. Until the tank is stable, go to half-strength EI. No need to add Flourish iron in addition to EI.
- Too much light is a key problem. Cut intensity in half or more. If you really want to turn things around, cut light by two-thirds.
- Not enough CO2 is a problem. Lots of threads on how to do this. Invest in a pH probe. Start the CO2 an hour before lights come on. Get a pH drop of 1.0 to 1.2 and keep the bubble rate so the pH stays there all day. You will need to add vigorous surface agitation with a Koralia type pump.
- Unstable CO2 may be a problem. Without surface agitation, it is difficult to do this. Yes, you lose some CO2 to the agitation, but the agitation will blow off excess and add O2 to the water, which allows fish to withstand higher CO2 levels. It's not entirely logical or minimalist, but it works.
- Excess organics is a problem. See suggestions below.
You have black beard and blue green algae - this happens when there is too much gunk in the water/substrate/filter combined with not enough or unstable CO2, dead or dying plants and all of the above compounded with too much light. Do more water changes. Vacuum every square inch of the substrate. Yes, that means removing and replacing plants. I know, people worry about this, but plants will bounce back just fine. Then clean the filter thoroughly. Then, cut and remove the bottom third of all stems and about half the oldest leaves. If you don't remove these old leaves, they will die and leach more gunk into the tank. By trimming older leaves and ugly stems, you are preventing blue green algae from coming back next month. Be brutal with the amount of trimming.
You have a light problem. Too much of it. It's like a stadium. Too much light will screw up any chance of you getting a balanced tank. You'd be shocked how easy things get when you reduce lights.
These are the basics.
Once you've addressed the basics, then you can start focusing on nutrient levels and ratios and such. Don't focus on nutrients yet because your pictures indicate that you need to address more basic stuff.
The things you want to increase are CO2 levels, filtration, surface movement, cleaning, and water changes.
The two things you want to reduce are light and ferts.
Once the tank has reached balance under low light, then increase ferts to full EI levels. We used to think high traces (micro nutrients) caused this problem. Not at all. EI level traces DO NOT cause this problem.
Light should be the very last thing you increase, if at all.