Lime is added to the medium..so it depends on what the ph of your medium is. Lime will only neutralize the ph, or keep it buffered closer to 7. If the medium is not very low, like at ph of 5 or less..then lime isn't necessary for now. As you reuse sphagnum peat moss..it breaks down..that's actually the yellowish color you see if you water to run-off...as it breaks down..it can get quite acidic, but the amendments in there will help with that for a limited time. If you use RO water, it won't make your potassium low as it doesnt usually come in high amounts with tap water anyway. You lose out on mainly calcium and magnesium as those 2 elements are usually highest in tap water.,,but it depends on your city and piping..some can have high amounts of iron and sodium as well. For future grows, you can even get silica as that's potassium only and has its benefits throughout the plant's life. With RO water..yes you need cal-mag by the time flowering hits for sure, as for potassium..try upping it a bit and lowering the ph a bit..if there is no fix, you may have a lockout issue instead because of an excess of another element. For those elements, you ideally want at least a 2:1 ratio of potassium to calcium and to magnesium, it can go higher, even as far as 7-1 for some strains. Phosphorus and magnesium work together, they get absorbed best at a 1:1 ratio. Too much calcium, magnesium, phosphorus or nitrogen, will lockout potassium.