This question and answer posted on the forum, is well worth noting if you have an under gravel filter (UGF).
Q: How do you folks feel about under gravel filter, I have not cleaned mine in 2 1/2 years. The mess under the stome is awful. The ammonia is fine pH seems normal. I am thinking to remove it and clean under there and wondering if I should put it back in.
A: There is nothing wrong with UGFs, but people either swear by them or swear at them. Personally I don't use them except for a little one gallon tank where most other filters are impractical.
Do you have the tank on a stand that allows you to see the bottom of the tank? Iron stands and many wood stands made for aquariums don't have a top on the stand so when you open the cabinet and look up you can see the through the bottom glass of the tank. If you have that sort of setup, I have a nifty trick that might work for you.
Get a long piece (like 6 feet) of clear tubing from your local hardware store. Not too big in diameter because you want it small enough to snake down the uplift tubes of your UGF and flexible so it bends fairly easily. Now find a ball bearing or a small nut. Silicone glue it to the side of the tubing at one end. You might be able to slip the nut over the end of the tube, but put a little glue to hold it in place.
Now snake the ball bearing end of the tube down the uplift tube until you see it under the plate when you look up through the bottom glass. Start siphoning out water. Now here is the nifty/fun part. Get a magnet and from the underside of the tank you can now direct the tube anywhere you want (because of the ball bearing) under the plate to get to all the built up gunk.
Don't worry about getting it all in one cleaning since you would likely have to siphon out all the water to get close. So just do this each time you change water until it is back in reasonably good shape down there.