Unless you use a fish tank for auxiliary purposes like for quarantine or raising fry, you definitely need aquarium substrate.
If you manage to avoid all of these pitfalls, soil provides the perfect substrate for live plants, and you will see rich and verdant growth. There is no need to supplement a fish tank with CO2 when using a soil substrate, and very little fertilizer is required.
You have several different options for saltwater aquarium substrates, and each has its own pros and cons. Aquarium hobbyists broadly divide marine aquariums into fish-only aquariums and reef tanks; each has distinctive features you need to take into account when selecting a substrate. Remember that no one substrate is always the "right" choice.
Substrates for goldfish tanks: Sand - The Goldfish Tank
planted fish tank substrate( dirt vs substrate) - YouTube
The bottom of the tank is one of the mostneglected parts of the aquarium, and as far as many aquarists areconcerned there really isn't much to consider. In a freshwatertank, you use gravel, and in a marine tank coral sand. If the tank hasan undergravel filter, you'll need a fair depth of the stuff, butif it's just a decorative covering to hide the floor of the tank,then you only need enough to hide the glass. So is that really all youneed to know about aquarium substrates? Definitely not! Choosing anunusual substrate is a great way to give a tank a distinctive look, andmore importantly there are many types of fish that appreciate specifictypes of substrate. Gravel and sand The aquarium substrate is generally the first step when setting up your empty freshwater fish tank. You must have some kind of substrate if you want to keep water plants, give your aquarium a natural look, and add driftwood, rocks, or .So with fish like catfish and spiny eels -- not tomention loaches, mormyrids, gobies, earth-eating cichlids, andfreshwater flatfish -- you really want to keep them in a tank with asofter substrate than gravel. Sand is an easy to use option, butaquarists do need to bear in mind that there are at least threedifferent types they are likely to encounter. Each has its uses, butbecause of their very different chemical properties they are not allequally suitable for any given aquarium. On the other hand, river sand cannot be used withan undergravel filter; unlike coral sand the grains are so small andcompact so readily that it cannot be separated from a gravel filter bedwithout impeding the flow of water. River sand can only be used as adecorative medium for covering the bottom of the tank, and in anunplanted tank the depth should be no greater than that which the fishcan easily move around while they're digging. The danger comes fromfood or other pieces of organic matter getting buried in the sand andallowed to decay anaerobically. Anaerobic decay is bad because itproduces toxic gases that can leak into the aquarium stressing, andpotentially killing, the fish. It is usually recommended thatsubstrates that are not part of an undergravel filter should be nodeeper than around 1 to 2 cm. Planted tanks can get away with greaterdepths, because the roots themselves will oxygenate the substratepreventing the risk of anaerobic decay, and where you are keeping bigfish, such as violet gobies or flounders, that can and do shift largequantities of sand, you can increase the depth of sand used accordingto the size of the fish in question.