don't worry about the nitrates.. most fish are fine.. the only thing i've found is that fry suffer when it's abover 400ppm
ph is ok, but i like mine a bit higher (closer to 7.2) just cuz.. and i supplement with maxicrop w/ iron as needed
shell grit might be a better option to buffer your system up a little, but you don't have to..
just keep an eye on it, as it may drop even more (that's why i buffer mine up higher)
your system is still in the cycling stage, so go light on feeding especially after you add the tilapia.. test daily at about the same time since some things will fluctuate..