One of the best strategies we have found for long-term enjoyment of the aquarium hobby is keeping it simple. As a fellow hobbyist I appreciate the excitement that comes when you see the vast variety of marine life available at the local fish store and online. However, such excitement must be tempered by the realization that you will have to spend time caring for that particular critter for at least months and possibly years.
There are some species there are notoriously hard to care for in aquaria because of their dietary needs. One example is fish that refuse to eat freeze-dried or frozen foods, and will only eat live foods. The most popular examples of these are dragonette fish, and wild seahorses. While these are interesting fish to have the burden placed on the owner responsible for feeding them can often lead to frustration with the hobby as it takes up an increasing amount of your time. If live foods cannot be reliably obtained and fed regularly the fish may suffer health wise.
Other animals that present feeding issues are predators. Predators tend to be intelligent, and many are fun to watch. However they make keeping an aquarium difficult. This is because predators tend to be messy eaters, (eels are notorious for this issue), and because they may eat their tank mates. It is also important to remember that not all predators are fish. There are predatory snails, crabs, lobsters, and mantis shrimp that can crawl around your tank and cause just as much havoc as those that swim around it. The limitations caused by adding a predator to your aquarium should be weighed against your desire to keep the animal.
Coral can also present feeding issues. For example, certain non-photo synthetic gorgonians require the feeding of fine particulate marine snow on a regular basis. This can cause both a high nutrient load in the aquarium and a great deal of attention from the hobbyist. Because of the demanding nature of these gorgonians they usually fare poorly in an aquarium and lead to frustration and wasted cash except in the hands of experienced and dedicated aquarists. Corals such as these should be avoided.
Coral that can live off of the light available in the aquarium alone generally require less work. However, like everything else in this hobby there are exceptions to the rule. Certain SPS coral require bright light and low nutrients that require the hobbyist to be very diligent about water quality, and to spend extra money on lighting and possibly cooling to deal with the heat given off by the intense lighting that is required. Until you are an experienced hobbyist you would be best served to refrain from keeping the species until you are more comfortable maintaining their required habitat.
Size is also a consideration when selecting a potential inhabitant for your aquarium. Not only is there an issue with large animals not being able to have enough room to travel, there is also the issue of their travel causing a mess. For example, urchins are excellent tank cleaners but because of their size when they move over rocks they can dislodge and knock over decorations in your tank that are not firmly secured to your rock work.
The last consideration to make is risk. Different aquatic species present varied risks to the aquarium. Some species present a risk when they die. For example, when macroalgae dies it can lower pH and release nutrients into the tank. Or when a cucumber dies they tend to be very messy and can foul a tank. Some burrowing animals have a tendency to cause rock slides, some macroalgae and fast growing coral can overtake a tank, and certain corals as - especially leathers - can release harmful allelopathic chemicals in to the aquarium stressing the other livestock. Sea hares disperse ink when they feel threatened, some fish have a venomous sting, while other fish are prone to infections that can spread (such as Tangs). The potential risks a particular inhabitant may have can be wide-ranging, but investigating the species on Google should yield enough information for you to make an educated decision about whether or not a particular species is better to view in someone else's aquarium or yours.
If you do decide to keep a fish or invertebrate that comes with risks, there are steps you can take to mitigate the harm that can potentially be caused. For example you can run extra activated carbon in your system in case allelopathic chemicals, or ink is a concern. Extra water volume can be added to your tank via a sump to reduce the harm a death can cause to the tank. You can wear gloves to handle sharp animals, or quarantine fish that are more prone to illness. The best mitigation strategy will depend on the nature of the risk involved. I hope this guide will help you make a decision that will keep you enjoying the hobby for years to come. Happy reefing!