Many people believe that invertebrates are only for mini or micro-reef tanks. Not so. There are quite a few invertebrates that do well in non-reef tanks. However, not a lot of invertebrates should be attempted by inexperienced saltwater fish keepers. Below is a brief summary of the more hardy invertebrates available to aquarists.
There are many different shrimps available in the market, with most of them being perfectly suitable for a lightly loaded saltwater tank. In fact, some shrimps are more suitable for fish and invertebrate tanks than for a reef tank since they like to eat corals.
Some of the more popular shrimps are Cleaner shrimp Lysmata amboinensis, Blood shrimp Lysmata debelius, Candy cane or Peppermint shrimp Periclimenes brevcarpalis, and Coral Banded shrimp Stenopus hispidus. The cleaner shrimp is denoted by a white on red stripe down the middle of its back. They are fairly inexpensive and easy to keep. They should, however, be kept in small groups (34), as this makes them more social and more likely to come out often. The Blood shrimp is intensely red with some white spots. It is a very striking animal, but usually commands a high price. The Coral Banded shrimp is very popular with reef keepers, but must be watched around small fish. This shrimp has been known to eat small fish without thinking twice.
Most shrimps are scavengers and don't necessarily need to be fed overtly (they usually eat food dropped by fish). If your fish your fish consume most of the food before it makes it to the bottom of the tank, then some extra food should be given to the shrimps after the fishes have been fed, or at night (most shrimps are nocturnal). Shrimps readily accept most frozen foods and dried foods (brine shrimp, flake food, etc.).
Stay away from Harlequin shrimps Hymenocera sp. as starfish are their only source of food.
There are many different type of crabs, but the most commonly seen varieties are anemone crabs Neopetrolisthes ohshimia, arrow crabs Stenorhynchus seticornis, and hermit crabs Dardanus megistos. Anemone crabs live in anemones, as do clownfish (e.g., Sebae), and vary greatly in color and shape. They are usually acquired indirectly by buying an anemone, but are some times sold separately. These crabs should have a host anemone to feel comfortable. Arrow crabs are very interesting animals which should be kept one to a tank, as they will continually fight. Also, Arrow crabs should not be kept with Coral Banded Shrimps as they will fight as well. Hermit crabs are also interesting, and vary in color and size. Most are passive, but some will eat corals and other invertebrates.
Crabs are generally omnivorous and readily accept the same foods as your fish. Like shrimp, crabs can only eat food which has made it to the bottom of the tank. Thus, ensure some food is in reach of your crabs.
Most sea urchins and Starfishes are suitable for beginners who have a few months experience. Once again they vary greatly in size, shape, and color. Beware, some sea urchins are poisonous. Most sea urchins and starfish feed on detritus and algae, and small particles of food that have fallen within their reach.
Simply put, anemones should not be kept by beginners (sorry folks). They all require very strong lighting and excellent water conditions. Do not believe a fish store guy that tells you otherwise. Unless you are willing to invest a lot of money in proper lighting, do not try to keep an anemone.
Invertebrates are very sensitive to water quality. Signs of stress due to poor water quality will usually be exhibited first by invertebrates. Therefore, shrimps, anemones and other invertebrates should never be used to cycle a tank. Moreover, you should never add an invertebrate to a diseased tank or a tank which does not have stable water quality factors (e.g., pH, temperature, etc.).
Other points to note. Shrimps need iodine to properly molt, as well as calcium . If you do not change water regularly (which you should), or if you do not feed live or frozen food frequently, then you may need to supplement your water with iodine. Without proper levels of iodine, shrimps will not molt properly and will most likely die. Also, copper kills invertebrates at much lower concentrations than fish. If you have ever used copper in your tank, DO NOT put invertebrates into the tank. You will never be able to adequately remove all the copper such that you can keep invertebrates alive and happy. Finally, crabs usually outgrow their shell sooner or later. Therefore, you will need to provide a new larger shell (they usually try a few out before sticking with one, so you will probably need at least a couple).
Tridacna clams (they need strong lighting), Flame scallops (they are nearly impossible to feed in an aquarium as they are filter feeders), Octopi (they have very short life spans), Nudibranchs (they are difficult/impossible to feed), any hard or soft coral (they need very strong lighting), and sea squirts (they can release poisonous toxins into the water).
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