Ammonium bicarbonate is more bioavailable. Potassium nitrate can cause potassium to rise too high over time if the NO₃ demand is high; sodium nitrate is better.
Corals and macroalgae consume phosphate and nitrate in a fixed ratio. If nitrogen is too low, phosphate consumption will stop. Raising the nitrogen can allow PO₄ consumption to continue, and without enough phosphate import, will decrease (and vise versa).
I just noticed your phosphate is also very low. That may be even a bigger concern than low nitrate. Here’s my phosphate dosing recipe. The link has the Amazon link to purchase it.
https://www.reef2reef.com/threads/diy-phosphate-dosing-recipe.1109919/#post-13548973
Buying phosphate and ammonium is substantially more inexpensive than neonitrate etc. In addition, the linked source is pure with no unknown byproducts.