Tap water varies all over the board, some people like myself are on well water, well water will almost always be "cleaner" (less ppm) than city water, city water can still be used, as deepwaterdean suggested 200ppm or less out of the tap is kind of the highest you want to see it at, anything above that and youll wanna get a water analysis done so you know what exactly is in it to determine wether or not you wanna use & amend it. Some people are a bit more.. lets say "picky".. about their water and go for RO to have a completely clean slate to start with. I do have an RO system in my house that i could use if i wanted but for my purposes its not worth it.. the 4x waste factor is a kick in the nuts to the earth and besides that the 80-100ppm that comes out of my tap is all micro nutrients that id just be adding back anyway. my suggestion is build yourself a little starter bubble bucket that you can start your seeds/clones in with just water, some rooting nutrients like dyna-gro's K-L-N for example are okay on the babies but as a general rule you dont wanna give infant plants too much "juice" at least until a few sets of true leaves are formed and/or you have a good rootball started. photos are of my old starter bucket. these plants were all clones, in rapid rooters, once i could see roots poking thru the rapid rooter i'd transplant it into a 3inch netcup with hydroton and only use ph'd water with K-L-N added per bottle recommendation. btw the 2 root pictures are only 48 hours apart
Edit: To answer your question OP yes raise ppm by adding more nutes, i've never used sensi myself but you say you have a grow 2-part and bloom 2-part, i'd assume, as with most nute lines, you will fade out/stop using grow and begin using the bloom when you switch the light cycle, you should be able to find a feed chart on their website to help guide you in the right direction.