Substrate in fish tanks refers to the gravel, rocks, sand, etc that is on the bottom of your tank. Certainly it increases your tank’s aesthetic appeal, but do you really need it? The answer = no.
“But it provides biological filtration!”
Yes, your substrate does hold some of the friendly nitrogen-converting bacteria, but to a significantly lower level than your filter. Since your filter is constantly receiving new water flow, those bacteria are constantly being fed their ammonia and nitrite requirements. Water flow in your substrate is severely decreased compared to your filter and therefore not a big component of your biological filtration. Without a substrate, your nitrogen cycle will not suffer. (Don’t have a filter? Then you have bigger problems.)
“My undergravel filter won’t work without substrate!”
Actually, an undergravel filter works better without substrate. For those of you unfamiliar with the concept, and undergravel filter lays across the bottom of your tank and pulls waste from the bottom of your tank directly to your filter. It’s a great concept, if you keep it clean. Unfortunately, most substrates will plug up the intake of your undergravel filter our only pull water from the sides of your tank where the current is greater, making them less effective.
“My fish use their substrate to scratch themselves!”
Ouch. Yes, we’re using actual quotes here. Usually, this type of scratching is known as “flashing.” It is a common behavior of fish suffering from a parasite infestation. Parasites on fish = bad. Get your fish diagnosed and treated and you won’t have to worry about them “scratching” themselves.
“What’s the best substrate?”
For most aquarium fish, typical gravel substrate is sufficient. It comes in lots of colors to suit your tankscape and is made specifically for fish tanks. If you have bottom feeders, such as catfish and loaches, they may benefit from a sandy substrate that isn’t so heavy to move. If you have a negatively buoyant goldfish, a smooth substrate, such as a bare bottom or glass stones will help protect the skin on their bodies in contact with the bottom of your tank. How your fish interacts with the bottom of their water column will determine what kind of substrate is best for them. Do your research!
If you are using “found” substrate, as in you took it from somewhere in the wild, NEVER use substrate that has been in contact with any other fish. You can try to bleach/bake, it but it will never be 100% safe. Soak any rocks/wood in tap water overnight. If you notice any odd patches of oil on the surface after the soak, toss it. You can add 10% bleach if it is non-porous.
“So a bare bottom tank is okay?”
Sure. Your fish probably appreciate a substrate and it will provide a footing for any decor. However, it is significantly easier to clean your tank. If you are going to have a bare bottom tank, be sure to provide decor your fish can interact with. It is rare that a fish will thrive in a completely empty tank. It is common to have bare bottoms in quarantine tanks in order to make them easier to clean. Quarantine fish are usually not feeling great from either being in a new environment or disease, so make sure to give them places to take cover.