Mike has good advice there I have a single lone tank (quarantine tank...) and in the winter the humidly is in the house below 12 percent. I use basically a plexi lid on the tank much like Mike has pointed out and I also use coco coil (eco earth) as a substrate it is great for humidity. I break up the block some and boil water and add boiling water (it sometimes moulds) and mix it up like that (let it cool overnight before using it...) use a little water don't soak it just enough to loosen the block up. It will hold for a few days like that maybe a bit longer. Over the cool side make a big deep mound of it and when the humidly dips again SLOWLY add some water (like a cup) to the pile (NOT over the UTH!!!) it will make a humid pile and that should be good for a while. I hope this helps!
Alex