Substrate is to an aquarium what soil is to a garden. It can include sand, gravel, crushed coral or crushed seashells. Some colored gravels should be avoided since the color may leach out into the water over time. The substrate should not adversely affect the pH level of the water. Crushed coral or shells and some types of gravel create a high pH and high carbonate hardness which is detrimental to the plants. Low pH and very soft water can cause root-rot, which will lead to algae growth. The ideal level is 6.8 to 7.2 pH depending on types of plants. Avoid synthetic materials, gravel that is coated with epoxy or glass beads. It is theorized that the bright reflection coming up from the bottom of the aquarium disorients the fish.