1) cycling still at 3rd day, so i dont think need to change water yet. you will disrupt the cycling process if u change out all the water.
2)IMHO i dont think it is necessary to buy nitrite test. just cycle water for 4-6 weeks can already
3)think your ammonia already high enough already, so dont think you need to add
4)i think that that is debris. it is up to you if you want to siphon it out or not.
5)your prefrence!
6)think you should remove stock filter media and replace it with more coral chips or biohome
7)add ph buffer
BTW how many fishes do you intend to keep?