Where are the drain holes on your pail, how many, and how large?
Your runoff is very low for the pH that you are watering with! How long since you last re-potted the plant? I think the lime in your soil is long gone and you are seeing problems from that, not just pH, but also too little available calcium.

Re-pot on a schedule that allows for fresh soil every 4-6 weeks.
And I think you are watering too often.

Edit- ventilation is just a fan, or do you have a correct EXHAUST as well?