You're right about the over-fert and Mg deficiency. The golden yellow spotting you see in between the leaf veins is Mg deficiency. You're also getting some leaf tip yellow and brown colors, indicating fert burn. So... looks like you're ferting with too much nitrogen and not enough Magnesium. For Mg, you can just use about a teaspoon of epsom salts in a gallon of water and water them with that. The plants look like there getting too big for their pots. It should be understood that a root-bound condition can kill leaves too, but exact appearance of leaves when this happens is unknown to me as I've never seen these plants in a root bound condition.
You might consider getting a more balanced fert other that 8-2-6. Also remember that your soil might come with fertilizer mixed in and if so, then adding more fert to a small plant might be too much. On your water pH, remember to shoot for a pH that is both water and any ferts mixed together. If you lower pH then add fertilizer, you have to check the pH again to see if it is where you want it. Chances are you can avoid using the phosphoric acid.