Algae
No matter how diligent and attentive an aquarist you are, it's fairly certain that at some point you will have an algae "problem" of some kind. "Problem" is in quotes because algae, by itself, is not harmful to your tank unless it starts growing over your corals and other sedentary invertebrates (or when the algae bloom is in-water and is disrupting light penetration into the thank). The basic common classifications of nuisance algae are "hair algae" (red or green), "turf algae" (any variety of short-growing, plant-like algae), "bubble algae" (actually does look just like bubbles and can be green, red or even black), "cyano" (which is actually the bacteria, cyanobacteria) and in-water column algae (turns the water green).
Every aquarist has a different level of tolerance when it comes to
algae. Some are perfectly happy thinking of their sizable patches of
green hair algae as nothing more than a peaceful underwater meadows.
Others are quite unhappy at the slightest bit of hairy green material
anywhere in the tank. In any case, the most probable cause of algae growth is excess nutrients. This doesn't always mean that your nutrients are at toxic or even less than ideal levels. It simply means that there's enough to support some extra algal life in addition to your desired tank life. To reduce the levels of nutrients in your tank, simply increase filtration and/or decrease feeding. Sometimes time alone can be the cure. Biological filtration can take some time to build up to match an excess of nutrients. This is especially true in younger tanks less than a year old.
Aiptasia
Aiptasia are small, brown anemones that plague almost all marine aquariums. They are probably the most annoying problem faced by reef aquarists. A few aiptasia are not usually a problem. However, they quickly multiply to plague numbers. These anemones, like any anemone, can sting their neighbors. They can grown on anything and everything and are nearly impossible to kill. Attempting to scrape or pick them off only makes things worse since fragments of them can spread throughout the tank and grow new aiptasia. The best way to prevent an aiptasia outbreak is to kill them as soon as you see them (even if it's just one or a few). Do not waste any time. To kill them, mix 2 tsp of NaOH (lye), with 2 tsp Kalkwasser (calcium hydroxide) in 100ml of distilled water. Turn off all the pumps. Using a syringe with a dosing needle, squirt the NaOH/Kalk. solution onto the aiptasia. Make sure to keep the solution mixed by periodically shaking the syringe. Be careful not to hit anything else with this solution as it will kill anything it touches. Keep the pumps off for an hour. WARNING: Be VERY CAREFUL with NaOH. It is extremely coarosive! It will also heat up temporarily upon mixing it with water. Mix it in a glass container and always use gloves when working with NaOH!
If your aiptasia are already out of control, killing them by hand may not be enough. Peppermint shrimp (Lysmata wurdemanni) are known for eating aiptasia. However, not all peppermint shrimp will eat them. If you have a serious aiptasia problem and you want to enlist the help of some peppermint shrimp, purchase about 1 shrimp for every 5 to 10 gallons of tank. This should give you decent odds that at least a few of those shrimp will be aiptasia eaters. If you're lucky enough to get a few aptasia eating shrimp, they can go a long way to solving your apatasia problems.
Coral Diseases
See Coral Disease
Tank Crashes
A tank "crashes" when something goes wrong and multiple animals suddenly and quickly start declining. A lot of different things can cause a tank to "crash." Some common causes are sharp, drastic changes is salinity, pH or temperature. Sometimes, the cause might not be so obvious. It might be the inadvertent introduction of an unknown toxic or foreign poison (ex: household air-borne pesticides). If the cause is an extreme rise or fall of a major tank parameter (such as salinity, pH or temperature). It's important not to try to correct the problem too quickly. On the other hand, if the suspected cause is a poison/toxicity issue, heavier-than-normal use of activated carbon and other chemical filtration can help along with generous water changes.
Equipment Failures
Tank crashes can also be caused my equipment failures. When pumps stop working, water flow and gas exchange stop with them. Heaters and chillers can malfunction causing potentially deadly swings in temperature. Therefore, it's a good idea to test all new equipment in an empty tank before using it. Always make sure that the equipment you buy is safe for salt water. It's also important to monitor your existing equipment. Heater settings often need periodic readjustment. Bulbs for lights should be replaced approximately once a year. Faithful cleaning will also help keep your equipment (especially your pumps) running smoothly.