I'm not sure if you are in your veggie state or flower, but The water in an open face container will bring your humidity up if you are in the grow state. But dont use the water method if you are flowering. The moisture will cause you more problems than you are willing to take on. Mold and mildew will form in and on the buds, and the taste is nasty. For the flowering humidity levels, you can recheck your CFM exchange and make sure you are exchanging enough air, but just pull enough to keep from having stale air in the room. there are fomulas on the internet that give you the correct Lighting, temp, humidity, and many other formulas to correct the problem. The slowing of exchanged air will hold more moisture in the room. If you can afford it, there are numerous types and styles of humidifiers on EBAY. Also you said something in the effect of the temperatures dropping at night or whenever the lights are out. A small therostatical heater can be put in the room and set on a low temp, just to where the temp will not drop under 60 degrees. Any lower temperature could be detrimental to the plants in a long period of time. If you do a little research on Google, you can just about find the solution to most of your growing problems. Reference books: Ed Rosenthal, Dale Gieringer, And Gregory Carter. I hope this will help.
Happy growing.