I have low KH now but in the past is was higher and I still had PH swings. The reason is simple. most of the KH in water is calcium carbonate or magnesium carbonate. Yes plants can consume the carbonate, but when they do that leaves behind the calcium and magnesium. These covert to hydroxides and push PH up. Also when plants are growing they consume nitrates converting potassium nitrate to potassium hydroxide which also pushes PH up. hydroxides have a much stronger effect on PH than carbonates. Over night the hydroxides latch onto CO2 other compounds in the water and stop pushing the PH allowing it to drop does back down to 7.Huge pH swings (7-9 during a 24 hour period) indicate that your water is deficient in bicarbonates (as measured with a low KH of <2). Since many plants (about 50% of submerged plant species) can use bicarbonates as their carbon source, a low KH is not ideal for a natural tank (i.e., no CO2 injection). Bicarbonates both prevent diel pH swings and provide much-needed carbon for many aquatic plants. I would add baking soda (1 teaspoon/gal) to get KH up to 4-6 degrees.
If you look at all iron products out their you will find all are chelates but not all chelates are equal. The chelated irons are Gluconate, EDTA, DTPA, and EDDHA. Iron gluconate is rapidly attached by bacteria causing the iron to rust and can cloud water. It lasts about a day but it is cheap. iron EDTA oxides when the PH is above 6.5. It can also cloud the water. I recommended Iron DTPA it is stable up to 7.5 and sometimes up to a PH of 8. Above that it cn oxide and cloud the water but most tanks are below 8. Iron EDDHA looks very promising. Stable up to a PH of 10! However there is a catch. It colors the water red. So of all of the Chelates available the most commonly used ones (Gluconate and EDTA) can cloud the water because they are not optimal for most tanks. UV light does degrade chelates but regular indoor or artificial lights don't have UV so aquarium lights won't degrade the Chelates.You have supported my recommendation for adding chelated iron fertilizers at night or just before lights go out. That's when light and a potentially higher pH would be less likely to degrade the chelator.
it shouldn't take much (100-200 mg chelated Fe/10 gal tank). To a 10 gal tank, I add a powdered quantity of FeEDTA that approximates half the size of a green pea. Does not have to be exact, but remember that iron is required in small amounts. It can be toxic to plants and stimulate algae at excessive doses.