Change in 1 leaf Color

My first thought was chlorine toxification.. if there is a possibility, check your water supplies online.. You could also temporarily change to bottled water to see if it helps. Good luck!

pH at 7.7 is not ok. To be honest i think your plant looks good on pH 7.7 lol

This, PH I don't this so, it's more of a case of tap water. As if it was PH you'd of seen signs from the start and all other plants would be like this. Each strain has it own immune system and will only take so much of chlorine before showing signs of sickness and causing it to focus on that problem and in the process not getting nute uptake as her priority. Healthy plants=positive nute uptake. Filter your water. Or all plants might come under the same prob, as I said I'm assuming you water all plants with same water so if it was PH then they'd all be effected. It's only showing early signs on the lower leaf that sickness will spread as the plant has less fight in her while trying to deal with contaminated water.