you should have some sort of drainage layer under the substrate to allow the substrate to drain. unless of course you are keeping some sort of aquatic plant in an emersed state, such as a cryptocoryne species, which love a nice wet soil. to drain it, you can run a piece of aquarium airline tubing to the bottom and just siphon off as needed. one note of caution, keeping a fogger on all day creates a ridiculous amount of humidity at the bottom of the tank. i ran an ultrasonic fogger in my white's tank all day and after a week or so, it started to rot my terrestrial plants. my crypts LOVED it, but my pilea's and fittonia's just started to rot away. needless to say, i no longer run it all day and i have had no problems. plants now grow well. crypts a bit slower, but that's ok, i have been growing those plants for years, so i learned to be patient...lol you can always get a cheapie timer from lowe's that allows numerous on/off cycles per 24hr period. that way you can run the fogger for say 30 minutes, then off for a few hours, then on again for 30 minutes ect. it might save the plants you have planted in your substrate.