Thats alot of light, the BBA is telling on you and is usually associated with low/inefficient C02, double check that, with that amount of light you will need good C02.
Why do you need the KH @5? if tap is 3 or more you are fine without adding a buffer, also if you are adding the buffer and testing ph to get C02 content, that would most likely explain the BBA. The test is off.
Trace, usually 5ml to each 20 gal seems to work good.
This being for a highlight C02 tank (which you have) running well.
Although I have a 10g that gets 3ml trace and 1ml Fe Chelate 3x aweek, ( I just recently bumped it up to 4ml trace 2 ml Fe).
adds a nice sparkle to the plants.
Also I add the mAcro's dry, simple, fast, effective! no need for testing.
As GD said I wouldn't worry about getting a P04 test, simply dose a set amount to a known volume of water 3x a week with consistancy and good C02, you should be fine.
Manually remove the BBA get the C02 & nutrient's up, it will stop it from growing, but will not remove it.
For simplicity's sake you could dose that 20g dry like this
Sunday:50% water change, Seachem alkaine buffer (get KH up to 5), ??? 1/4Tsp Nitrate, 1/16Tsp phosphate, and or (1/4Tsp potassium once a week or 1/16 or so Tsp 3x aweek.
Monday:5ml Flourish, 1ml iron chelate solution (1 tablespoon added to 250ml)
Before you started dosing this rich, I question the plant mass of the tank!
Plant heavy and you will be fine.