There are several things you need to keep in mind when changing your aquarium gravel. With proper preparation and careful execution, you can make the switch without harming your fish.
1. Determine the Appropriate Kind of Gravel for Your Fish
There are kinds of fish that actually prefer dark or light gravel, or that like to camouflage themselves on it. In general, however, most fish don't care what color their gravel is. There are natural and bright colors for gravel, from sandy browns to orange, purple, blue, and multicolor rainbow mixes. The color preference is really up to you and will not have any substantial effect on the fish, though you will need to be more careful when choosing what material your gravel is made of. Some types of gravel are only suitable in freshwater aquariums, and there are types that are only recommended for saltwater aquariums. There are kinds that are specially formed to help your live plants thrive. There are some, like coral, that actually influence the pH of the water.
2. Plan Far Ahead
Clean your aquarium filter two weeks ahead of time. The day before gravel replacement, do not feed your fish. Also, test the water for nitrite and ammonia. Prepare your holding tank and rinse out the new gravel well before you begin the transfer process. Collect new water for the main tank and let it rest for 24 hours to let it age, to warm up to room temperature, and to help the chlorine dissipate.
3. Prepare the Gravel
Be sure to buy the appropriate gravel for your type of fish and water. Get enough to form a three-inch thick layer of gravel at the bottom of your tank. This usually translates into a pound of gravel for each gallon of water. A good way to rinse the gravel is by cleaning in batches, using a sieve over a large bucket. Rinse several times until the water flows clear. Remember never to add any detergents or other chemicals to the water, since they could potentially harm your fish.
4. Have Everything Handy and Work Quickly
Before you start moving your fish, you will have to turn off the main aquarium filter. It's not good for it to be off for a very long time, so make sure you have a holding tank for the fish next to the main aquarium. It's a good idea to fill the holding tank with water from the main tank so the fish are already acclimated to it. Transfer the fish and live plants to the new tank and cover it. While they are waiting there, add enough water to the main tank to let the filter run. After turning the filter back on, remove the old gravel from the main tank. Then install the new gravel and replace the live plants and decorations. Lastly, bring back the fish.
5. Monitor the Health of Your Fish
For the first day after the change, cut back on the amount of food you feed the fish. Also, darken the room near the tank to reduce their stress. You should check the ammonia levels in the tank about three days after the gravel change. Check again in three days, and check a week after that. If at any time the ammonia levels are high, change the water and test the ammonia frequently until it is cleared out.