Brown spots on lower leaves

deficiency, if only feeding/maintaining 250-275. A lot of tap water comes out at 250-300. A modest feed is still 600-700 + tap and perfectly safe for all stages of plant growth -- more likely if there isn't a cartoon graphic on the label anyway. low on plant, so it is mobile. That's an early leaf, so it may not be neccessary to worry much unless you see it spread. The plant could just be getting rid of it. Since other leaves look healthy, i'd say a K-def if anything, but it shouldn't be impacted by the pH too much. If you've been giving a significantly reduced feed on top of what you see, then that's almost certainly why you see the leaf problems. as far as pH, that's from the constant evaporation. you'll find a happy medium by adjusting up and down. As long as you maintain the same brand feed, you'll likely need just one or the other and a consistent amount per day or week etc if consistent otherwise, but good to have something to raise and lower pH readily available. Vinegar can work as a ph-down, for example. I wouldn't use sodium-bicarbonate, but you could use that to pH-up a time or two and not have any worries at all about the added Na+. Over time it will cause a problem, especially if you re-use the substrate. In a hydro setup, you are 100% responsible for that. In soil the microbes help maintain and buffer any large swings due to what you put in.