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Sand as a substrate has many advantages over gravel. Sand is more natural, easier to clean, and looks much better.
Sand is Much More Natural than Gravel:
Almost all the fish we keep in aquariums are from waters that naturally have a flow much lower than would allow gravel as a substrate. Most will have a substrate of sand, some even silt or mud (which we can’t really have in an aquarium). Many fish like to sift through the substrate looking for food. This behavior is allowed for much better with sand instead of gravel. In addition, many fish like to dig pits. Sand is more natural for these species, as well as easier for them to do this. The piles and slopes they create with a smaller sized substrate are not as steep as they would be if they have a larger substrate such as gravel.
Sand is Much Cleaner than Gravel:
Sand is much cleaner than gravel. There is much more space between pieces of gravel, enough to allow debris in. The debris can buildup which in time can break down and rot, which increases nitrate and phosphate and lowers water quality. The space between sand grains is not enough for a significant amount of debris to get in. This keeps the debris on top. If there is enough flow in the tank the debris will keep moving until it is collected by the filters. This means less cleaning and a cleaner tank. If there is not enough flow then the debris collects in a few spots that are easy to vacuum out during weekly water changes. In fact sand is so little maintenance I had to STOP using it on clients' tanks because the maintenance was too easy. They saw the amount of work it took and some decided they would do it themselves. I actually had to start using gravel again in order to make maintenance jobs more work.
Estes Marine Sand (aka Stoney River and Ultra Reef):
This is in my experience the best sand on the market. It is extremely uniform in size, and the perfect size. It is large enough to allow oxygen to get to the entire sand bed (even with 3 inches of sand), yet small enough to keep all the debris on top. It comes in multiple color options. I usually do a half black and half white mix. This is is not too dark, but not too light (fish will lighten their colors if on a light colored background or substrate). The mix hides anything that may fall to the bottom. Estes requires no cleaning or rinsing, just pour it in. It is also inert. Since it has a polymer/ceramic coating the silicate base is completely sealed off. It will NOT alter the pH, hardness, add silicates, or do anything. It also costs about the same as gravel so the price doesn't have to weigh in to the decision at all.
There are other aquarium sands out there but I haven't talked to anyone who has had the same experience with any of them that I have had with Estes (no cleaning/rinsing, no maintenance, no gas pockets, etc.) so I cannot recommend them. Even people who have used other aquarium sands can have a lot of trouble believing me when I say it really is not work at all, ever.
What About Toxic Gas Pockets?
Many people have a concern with pockets of anoxic bacteria developing, which can give off toxic hydrogen sulfide gas. In my experience this is an issue having to do with the type of sand used, not sand in general. Many people prefer to save a few bucks by using cheapo sands that are not made for use in aquariums (play sand, pool filter sand, etc.). These sands are not as uniform in size or the right size to allow enough space between grains of sand for oxygen to get to the whole sand bed. The lack of oxygen they can create is what allows hydrogen sulfide producing bacteria to thrive. This is what can cause a rotten egg smell. In some extreme situations this can actually kill fish.
What About the Silicates Causing Brown Algae?
Another issue with cheapo sands is that they are silica based which when placed in an aquarium have been known to cause brown algae outbreaks (because of the increased silicates) that were only fixed by taking the sand back out again. Some people do not believe this is the cause, but the people I have talked to have converted mature tanks (outside of a new tank's algae phases) that were not having problems. They took out the gravel, added the cheapo sand, and had more algae than they had ever had before. They tried other fixes but nothing worked until they broke down and took the sand back out. This may not prove it to some people, but to those who have experienced there is no doubt.
Won't Estes Marine Sand Raise the pH?
For certain tanks where a high pH and hard water are appropriate I would use crushed coral in the filter (bagged, just like carbon). True marine sands can alter the pH but then you run in to the same issues as many non-aquarium sands (wrong grain size, grain size isn't uniform, etc.). Coarser marine substrates like crushed coral will just trap a lot of debris, just like gravel, so it is still more maintenance than necessary.
What About Sand Getting in to the Filter?
Sand use with filters is a concern for many people. Estes’ Marine Sand is relatively heavy and falls very quickly when fish have a mouthful and swim off with it, slowly dumping it along the way. It drops very quickly. The fish literally have to spit it in to the filter intakes for there to be an issue. To be extra safe I keep all filter intakes at least half way up the tank.
Obviously, what looks best is a personal preference. I find sand to look much better than gravel. Tanks that I have seen that have sand as a substrate seem to be more natural and look better overall. I have seen tanks that were very natural otherwise, but the gravel used (although it was average size and natural color) was actually noticeably unnatural, which can be distracting.
Sand is also a better grain size for plants to root in. I have used the Estes' sand very successfully in planted tanks. There is not anything in it to feed the plants, so I just use root tabs. Specialized plant substrates will eventually become exhausted anyways, meaning you have to start using root tabs or change out the substrate completely.
One of my favorite uses for this sand is in my reef tank. I personally do not like the bright, sterile looking white sands usually used in reef tanks. They also show off anything that happens to land on the bottom. I painted the back of my reef black and used solid black sand. The look was amazing. All the colors looked great, what little algae did develop didn't stand out, and anything that landed on the bottom did not stick out like a sore thumb.
Some species of fish not only do better on sand but require it. I personally would not keep goldfish on anything else. In nature they sift through the substrate in search of food all the time. And seeing how much they do it in an aquarium it is obvious that this is a very natural behavior for them that they should not be denied. Even better is that they help keep it clean for you. Other species like axolotls (a salamander that stays larval/aquatic its whole life), saltwater sand-sifting gobies, and others simply will not do well on other substrates.
In my opinion, in ten years or so gravel will be looked at the way under gravel filters are looked at now: old fashioned, not ideal, and the only people still clinging to them are the people who have been using them since they were the standard, figured out how to make it work well enough, and still stick with it.