Mixing Fish and Inverts
Although invertebrates and fishes may live in the same habitats in nature, they should be mixed with care in the closed environment of the aquarium.
In recent years, the number of freshwater invertebrates available in the aquarium trade has steadily increased. The popularity of these and the large number of freshwater invertebrates in nature suggests that there will be even more in the future.
Some crabs can be kept correctly in an aquarium with fishes, although many require special conditions and can only be mixed with certain species. Crabs can present a number of problems in the aquarium; most require brackish conditions and all are experts at escaping. Crabs can also be destructive, causing damage to plants and digging in the substrate. Ideally, crabs should be kept in a brackish aquarium with a sandy substrate and plenty of hiding places. Avoid mixing with slow, bottom dwelling fish, which are likely to be nipped. Crabs are natural scavengers and explorers and will eat almost any source of food. If they are kept in soft water they will need to be fed specialised food containing calcium, which the crabs use for creating their shells. Because they are adept at escaping, the aquarium must have a tight fitting lid with no holes. In case of escape, a good tip is to place a wet sponge on a plate on the floor near the aquarium. If any crabs escape, they will head to this moist area to prevent drying out, they can then be replaced in the aquarium.
There are a number of freshwater shrimps, although most are hard water or brackish species. Shrimps can be very useful in the aquarium, they are natural scavengers and algae eaters and very few are destructive. The biggest difficulty when keeping shrimps is to avoid mixing with fish that are likely to eat or nip at the shrimps. When carefully mixed, shrimps can be excellent aquarium additions. A species worthy of note is the Japonica shrimp (Caridina japonica), which are excellent algae eaters and a helpful addition to any planted aquarium.
In most cases, snails are a pest, and many fishkeepers spend great effort in eradicating them. The problem is that many snails can breed very quickly, over-running the aquarium and becoming quite unsightly. There are a number of methods of eradicating snails, one of which is chemical treatments. It is worth noting that such treatments are quite strong and may have side effects in the aquarium. The best way to control snails is to use natural methods, such as snail eating fishes or snail traps. Large snail species are less of a problem in the aquarium, if these do breed, they grow quite quickly and so are far easier to spot and remove. The Apple Snail (Ampullaria sp.) is a popular aquarium snail that can grow up to 10cm in diameter! These snails do not have many care requirements, but they should not be kept with large, destructive, or nippy fish, which may harm the snails. When keeping such large snails it is important to check them daily - a dead snail will quickly decompose, causing water quality problems.
Crayfish & lobsters
There are a number of problems with keeping freshwater crayfish and lobsters. Crayfish in particular have become a big problem when accidentally or purposely introduced into wild waterways from which they did not originate. When this happens, they can cause widespread damage to the natural environment and even kill native species. Because of this fact, it is vital that any fishkeeper keeping such species acts in a responsible manner and keeps the crayfish in the aquarium. Besides this point, many crayfish and lobsters will happily eat any fish that venture too close. The best advice is to do a little research first, find out the best way to keep such species and only keep them with suitable fish species. Any retailer happy to sell you a crayfish or lobster without checking your aquarium is suitable should be avoided.
The small Xenopus species of frogs are often seen in aquarium shops and are quite popular as 'something different' for many fishkeepers. Although these animals have been kept in community aquariums successfully, like any animal, unless you have done your research they should not be kept. Furthermore, your retailer may well be very knowledgeable on fish care but it is unlikely he/she is an expert on frogs. The Xenopus species are true aquatic frogs and whilst they still need to take air from the surface, they do not need, or want, for above water areas. The frogs are scavengers and will do well if fed on plenty of frozen or live foods such as bloodworm.
Any other frogs apart from these species are likely to require dedicated vivariums with carefully controlled humidities in above water areas. Much like crabs, most frog species are also good at escaping so make sure any holes are well sealed, ideally with a mesh that will still allow plenty of ventilation.
The Map Turtle, and similar species have appeared with increasing frequency in UK aquatic shops in recent years. These small reptiles will usually grow no bigger than 15-25cm (6-10 inches) but they are completely unsuitable for mixing with aquarium fish. As well as being capable of eating fish, they are also destructive, messy, and require specialist reptile care, which cannot be advised correctly by the majority of aquatic stores who sell these species.
When we purchase our fish from a specialist shop, we should expect to be given expert advice on their care. This does not always happen, and most fish keepers will have to do a little shopping around to find a good shop with well trained staff. Being an expert in aquarium fish however, does not mean you are also an expert on lobsters, crabs, turtles, or other aquatic species. If your local shop is stocking such species without being able to give you accurate and expert advice, you should start shopping elsewhere. The final word is: Do not purchase any species without looking up its care requirements and nature, the result can be disastrous and cruel to the species being kept.