This is one of the most common but most easily preventable problem in growing cannabis, especially for newbies who tend to go too hard trying to give their marijuana plant everything it needs like a first time mother.
One trick is to weigh your plant dry and then weigh it watered. You should water again when about 80% of the water weight is gone. Never let water puddle in the soil or on leaves or you’ll cause mold, bugs and more bad stuff. The soil should almost be dry before rewatering and it will usually be a couple days in between watering.
Add perlite to make your soil more airy, or add hydrogen peroxide for extra oxygen to be added into the water and to kill existing bacteria. If necessary, buy a moisture meter. Your leaves will appear generally droopy when over watered because your roots are suffocating! Though the roots suck up water constantly, the roots need to be exposed to fresh air every now and then.