Q. I would like to set up a seahorse tank, but I am confused about live rock and live sand. Some people say you need it and others say it is bad. What do you think?
A. What you have heard about live rock and sand are both actually true, but the need for live rock or not depends on the type of seahorses you are planning to keep. If you are planning to keep the larger species of seahorse such as the Lined seahorse (Hippocampus erectus) or the Brazilian seahorse (Hippocampus reidi) then live rock can help support your tank.
Live rock offers its benefits from the biological bacteria that it carries. This biological filtration is a very important process of your tank that helps get rid of and breaks down fish waste. In other words, biological filtration is a natural process in which nitrogenous waste products are converted into relatively harmless byproducts. Ammonia is broken down by a population of Nitrosomonas bacteria that is converted into nitrites. Nitrites are then broken down by the Nitrobacter bacteria which is then converted into the less harmful substance, nitrate. Nitrate is not dangerous unless it builds up to excessive levels and this can easily be prevented through regular water changes. However, you do not necessarily have to purchase actual "live rock" to help establish your biological filtration. You can actually purchase "dead" or dry rock and in no time at all it will become populated with a multitude of organisms, and you will have created your own live rock. One thing to keep in mind when selecting rock is to choose rock that is very porous — this offers more surface space and, therefore, an increase in the beneficial bacteria.
Along with the good, the bad and the ugly can also accompany live rock. Rock anemones (Aiptasia) and hydroids are the biggest and ugliest threats in that regard, but generally just affect the dwarf seahorses (Hippocampus zosterae). Although you will probably not see any hydroids on the rock because they are generally very small and initially very few, once you start adding the dwarf seahorses' main staple diet of brine shrimp, they will often appear quickly in epidemic amounts. The constant supply of the baby brine shrimp provided for the dwarf seahorses also creates the perfect environment and enrichment to help Aiptasia and hydroids colonize and grow, and these stinging animals can kill your dwarfs, especially their fry. However, this does not mean that you cannot have rock if you enjoy the looks and benefits of rock. You can make use of the "dead" or dry rock in this instance and, before you know it, you will have your own "live" rock, free of critters that may be harmful.
Live rock is valued by many people in the hobby, but there are those who choose not to use it. The choice of course is ultimately yours, but I will mention that the utilization of good mechanical filtration, a skimmer, and substrate such as sand will also aid in the breakdown of waste. Just take into consideration your final goal and of course the type of seahorses you want to keep, as well as the risks and benefits. Just remember, what works well in one tank may not in another. If you have an established system that is working well for you and you are happy, then there is probably no need to change things, but if you are new and need some guidelines, then hopefully this may help you achieve your objective.