I'm just going to throw this out there. I have somewhere around a 5% chance of being right. This keeps happening every time you have grown for the last few times, and always about a month or so into the grow. I think it could be bugs. It looks like it's working up from the bottom. If you leave just a breeding pair that gets back to your new grow, they will continue to multiply and the problem begins again. Sometimes they're so small you can't see them with the naked eye. If you don't have a high powered magnifying glass/microscope you can get them cheaply at Radio Shack or Amazon as examples.