While the scientific classification of corals can be a bit confusing, there are some easy terms that hobbyists use to help group and describe different types of corals. At the most basic level, there are soft corals and stony corals. 

Soft Corals
These corals lack a rigid skeletal structure and are 100% soft "flesh" like material. This includes leathers, nepthea, sinularia, and gorgonians.
Stony Corals
Corals that grow a rigid calcium carbonate-based skeletal structure. This includes both LPS and SPS corals.

Euphyllia Corals

Even with a clear definition, some LPS corals may still appear to be quite soft. This is because they are LARGE polyp stony corals and that large polyp will extend from the skeleton and may even flow in the water's currents, appearing fleshy and soft.  SPS corals or small polyp stony corals are different in that they appear to be rigid with many small polyps that simply look like colorful dots at a glance.  Up close, they are tiny fleshy polyps that extend from the rigid central skeleton. In either case, both LPS and SPS are considered stony corals because of the rigid central skeleton that contains the polyps. 

Soft corals do not have this rigid calcium carbonate-based structure and instead, they contain sclerites. Sclerites are tiny needle-like structures within the tissue that help support the structure. If a stony coral dies, the rigid skeletal structure will be left behind after the polyps whither away.  If a soft coral dies, the flesh will rot away leaving nothing behind except a small pile of sclerites. 

  • LPS: Large Polyp Stony Corals
  • SPS: Small Polyp Stony Corals
  • Soft Corals: Soft corals
Soft Coral
LPS Coral
SPS Coral

Stony corals uptake calcium and carbonate from the saltwater to create this rigid skeletal structure which is exactly why we have to maintain calcium and alkalinity levels in reef tanks containing stony corals. Soft corals on the other hand do not uptake calcium and carbonate from the water. Soft corals thrive under low-medium light conditions and typically do not require substantial amounts of flow. For these reasons, soft corals are often considered "easier" or lower maintenance because maintaining the environment is simply not as demanding as it would be in an aquarium with stony corals.  

Within the stony coral group, LPS corals are considered easier than SPS corals because they tend to be a bit hardier and their demand for calcium and carbonate is not as much as SPS corals. LPS will thrive under medium light conditions and require moderate amounts of flow while SPS corals are the most demanding requiring high-output lighting and considerable flow.  Most hobbyists start out keeping soft corals first, then try a few LPS, and finally SPS once they have gained the necessary experience.  

What Kind of Corals Should I Choose?

Technically speaking, you can keep soft corals and stony corals in the same aquarium, this is what we call a mixed reef.  While this lack of limitations can be very attractive when it comes to choosing livestock, a mixed reef is actually the most difficult type of reef aquarium to keep. Maintaining an ideal environment for soft corals, LPS, and SPS in the same tank is not an easy task. 

For the most part, it is best to choose a particular type of coral and build the aquarium around those corals' needs. Soft coral tanks can be put together on a budget and won't require the same lighting and pumps you might choose for an LPS or SPS reef.  The same goes for LPS vs SPS, the requirements are different and the equipment you choose will be different. This same idea applies to maintenance and dosing; you won't use as many additives in a soft coral tank when compared to stony corals and the maintenance routine will be different.  Choose according to your experience, budget, and available time to invest in the tank.

A new aquarist with a family, a busy career, and limited time to take care of the tank should probably only consider soft corals with the end of goal of some easy LPS down the road. If you have already learned the reefing ropes and are ready for a challenge, the mixed reef or SPS-dominated tank may be your magnum opus.