okay, so, if anything, delving into a planted tank has made it very clear to me why I flunked out of Chem in University lol.

I would really appreciate some clear, simple, basic dumby-english advice.

The tank in question is 90gallons, and has been up and running with fish for 2 1/2 months. I've had plants going for about half that time. Recently upgraded the lighting, added a DIY CO2, and am seeing a fair bit of success.

Substrate is sand
Lighting is 136 W T5 HO (three bulbs, two ballasts), and covers the tank from front to back and end to end - no dim spots that I can see.
pH is 7.0 (went down from 7.2 when I added CO2)
GH is 80ppm
KH is 40ppm (up from 10ppm since I added CO2 - I don't understand that at all, but I tested it twice both times, and those were the results)

Prior to a couple days ago, I was only dosing micro ferts once or twice a week, and not testing Iron. Nitrates were consistently at 0.0, despite a moderate fish load (6 angels, pair of breeding kribs, pair of nannacara, 5 glowlights, 3 platys, 2 yoyo loaches, 2 SAE, bristlenose pleco). I change 10-25% of the water weekly.

Tuesday, I started dosing with a pre-made liquid macro fert made by Hagen. I know this is not the best way to go (expensive!) but want to keep it as simple as I can, until I've got the hang of it.

Anyway, I've been dosing daily, and testing the water daily. Also started dosing the micro ferts daily, and testing water daily. Figure I'll keep that up until I reach the target ranges in the sticky on low-tech EI dosing. Is this a good approach?

After two days of dosing (haven't yet today), this is what I'm reading:
Nitrates 0.0
PO4 0.75 (up from 0.25 out of the tap)
Iron (both chelated and free) 0.0

So, should I continue daily dosing and testing? Is it normal for the phosphates to go up, but to see no change in the nitrates? Am I going to quickly reach an imbalance (too much phosphate not enough nitrates) using this ready-made mix?

fwiw, my algae problem (which was minor to start with) is definitely going away. My javamoss is starting to look green again (it was coated in a brownish grey, dusty-looking algae). Plants are all doing well, particularly the bacopa, vals, lily, various hygros, rotala, and ludwiga. Crypts, javamoss, anubia, and ferns are all growing much slower.

Fish all seem fine, except for one of my loaches, who is not doing well, although no visible signs of disease. I suspect, in the case of the loach, injury or stress caused by the male krib guarding the spawn site. Last time the kribs spawned, I lost a loach (the kribs aren't particularly aggressive, but the loaches are always trying to get into the cave, and get driven back).

I'm not much of a chemist either, but I still think the simplest way to dose fertilizers is to follow the EI method, and dose KNO3, KH2PO4, and a trace or micro mix. With T5 bulbs, and I assume with each bulb having its own reflector, you are in the range where you could follow a standard EI dosing schedule. You can buy the two chemicals at Aquarium Plant Food - hobbyist taking care of hobbyist … or http://www.bestaquariumregulator.com/ferts.html, and if you prefer to use liquids to dose, you can use Flourish as a trace mix. In any case you need to provide the plants with nitrates, phosphates and potassium in order for them to grow well.

For that size tank you need to use 2 or 3 DIY CO2 bottles, with their start dates staggered, in order to get enough CO2 and to keep the amount relatively constant. Don't worry about the pH of the water - CO2 will make it drop, but the fish will not be affected. In fact there is no good reason to test any of the water parameters except on rare occasions.