Yellow spot on my leaf. :) PH fluctuation ?

Aim for a ph around 6 this way if it drop it will dropbin acceotable range. Ph tends to swing if temps swings but not this much. Also check your ph after your mix your fert. Fertiliser wil drop ph. So if the water is at 5.5 and you add fertilizer it will drop a 0.8 to 1 easily.. i suggest putting ph buffer instead of changing everything. This low ph will eventually kill the plant du to nutrient intake problemes.