Any particular reason you're using RO
water instead of tap? As others have said if you use RO
water you need to replenish it with minerals such as Seachem Replenish or Kent's RO
Rite. As for the PH, try adding a few teaspoons of crushed coral or crushed argonite (sold at most fish stores in the saltwater section) into a mesh media bag or clean nylon stocking and put it in your filter (if it'll fit; if not just put it into the tank near the filter intake). Keep testing PH and remove/add coral as needed until you can get your PH up to around 7.
Also 10 days is a bit on the early side for nitrite. Typically it'll take 1-2 weeks to see an ammonia drop. Once ammonia starts converting then you usually see nitrites, typically in week 3 or 4. But with your PH so low and the RO
water not having minerals I'd fix those two things first for the cycle to even progress.