Sand, fine gravel, or coarse gravel?:

I am a big fan of small (fine) gravel or coarse sand. Food particles easily slip between the cracks in large gravel, where they rot, out of reach of most fish. In the tanks with finer gravel, the food sits on top of the substrate so that bottom-feeders can pick it up later and munch. Chicken grit and pool filter sand are great sources of cheap gravel for large tanks. Chicken grit is usually made locally, and may have minerals or shells that will increase the pH and hardness of water. Pool filter sand is my favourite - it has a very regular particle size and doesn't harm the barbels of my catfish. The only Corydoras sp. with great looking barbels in my house are on catfish who live in tanks with pool filter sand.

If you use an under gravel filter, you will be limited in the size of gravel you can use. If it's too fine, it will slip between the holes in the filter plate and clog it up (some people put a layer of open cell foam between the filter plate and the gravel to prevent this, but the foam can clog from fish waste, so you need to vacuum the gravel regularly). If it's too coarse, you will not have much surface area for your nitrifying bacteria. Also, consider running your under gravel filter in reverse. To do this, buy a power head that sucks water in and pushes it down the tubes into the gravel, instead of the other way around. By doing this, you force water up through the gravel, not down into it. This may help keep the gravel and filter plate from becoming clogged with fish waste and goo.

One other note: do you need gravel? Most people think tanks look more natural with gravel. Many fish breeders choose to use no gravel - it makes the tank easier to clean (no nooks and crannies for uneaten food and fish waste to collect in), and it does have a certain appeal - you can see your fish reflected in the glass below. Just think about it. Some fish love gravel and will feel insecure without it. Choosing the right gravel requires knowing a bit about your fish.

Colored or natural?:

Think about this one. When I set up my first tank, I bought red and blue gravel. I thought it would look nice with my orange and black goldfish. Now I'm stuck with the hideous stuff forever. Which do you want to catch your eye, the gravel or the fish? Look at tank in pet stores and books. Which tanks grab you - tanks with bright gravel, plastic plants and underwater bubbling scuba divers, or tanks with tan/brown "natural" gravel, some driftwood and green plants? To each their own, but make the decision carefully and think - "natural" goes with everything. Do make sure that the gravel you buy doesn't contain any minerals or shells that will increase the pH or hardness of the water, unless that is what you want. Gravel with crushed coral, shells or some kinds of rock will do this. To test it, you can drop a few drops of vinegar or other acid onto the gravel. If it fizzes, it will probably increase the pH of neutral water. You can also leave two cups of water on the counter for a few days - one with just water, one with gravel and water. Test the pH of both to see whether the gravel changes anything.

Rinsing your new substrate:

No matter what kind of substrate you get, make sure to rinse it well before adding it to a tank. To rinse new gravel, put about a gallon of gravel in a 5-gallon bucket, and run cool water into it. Stir and mix the gravel well with your hands, until the water is cloudy and silty. Pour the water out carefully, being careful not to lose your gravel. Refill bucket and repeat. This is easy to do outside with a garden hose, or in the bathtub. When the water is no longer silty or cloudy, drain the gravel, pour it into another bucket or into your new tank, and start again. Rinsing gravel is tedious, but it will greatly improve the clarity of your water.

Some credit goes out to:

http://www.aquariumfish.net
http://www.Fishgeeks.com