I'm with the 18/6 preference. MJ is like any plant in nature. Other than in Alaska and Siberia for a very short time, there's nowhere it has known any such thing as 24 hours of sunlight on earth for tens of thousands of years. The idea is you absolutely can do no better than to replicate it's natural environment artificially indoors to get the best results. Likely a pearlite and compost mix is a little hot with the N for such early stage seedlings. And it's also likely your mix medium is a little too acidic even though you're using 6.7-7 PH water. Unless you're saying that's the medium's PH level. The WW is more sensitive to too much N and low PH which leads me to make that call. Daytime temps s/b 75-80 deg. F and 60-65 nights. When seedlings are so small air circ. is hardly a problem, but the fan is a good idea anyway. Eventually you'll want a full air xchange in your room, tent, box or whatever you're using when they get bigger. After at least 4 weeks to get them past the seedling stage go to 12/12 to induce flowering, which will begin to show in about 2 weeks or less after the drop in light hours. As I've told others, once you go to 12/12 don't ever interupt the dark period even for a moment to peek at 'em. As this will cause a delayed setback for a few weeks more for them to finish. On another level some seasoned growers intentionally use this as a tactic to get longer stretched buds on their plants that once filled in make for massive long colas and significantly larger yields even though it takes longer for them to finish. For a way to get some of these results and more closely mimic mother nature, while still in veg mode I gradually phase to 12/12 over a two week period in three steps where I shave an hour in the morning and evening every week until they're at 12/12. The plants more gradually adapt which forms a longer main cola like plants grown outdoors. I use a scaled-up SOG technique and get very good yields by this method.