Brown spots on leaves. Can this be because of too high pH of the water (7.5)?

possibly, and you'd want to get that in order first before playing with nutrient balance... pH change will nullify many changes. Get that stable then see if it corrects itself before making any drastic changes. Also, water entirity of substrate. they aren't too large. So get a little runoff to ensure it is wet. wait for it to mostly dry before repeating. This may also be related to symptomes oyu see -- as well as pH swings... This habit change is important. Early on, it may take 9 days between.. no problem. easier for you.