Undergravel filters are counter-productive in tanks over 10 gallons, because you have to take everything out to clean under them. Just get a gravel vacuum-siphon tube and you can put in as much gravel as you want. Be sure to change 25% of the water every month. If you are using alot of gravel with live plants get an iron supplement too. Also, check out the nitrifying bacterial cycle on the web to see how an aquarium can become self-sustaining without having to break it down EVER.