Compared with the sea, the variety of invertebrates living in freshwater habitats is rather low. Insects are the most important freshwater invertebrates, but these are never traded as aquarium residents. Firstly most are only temporarily aquatic animals. Many species are aquatic only during their larval phase, become terrestrial insects after metamorphosis (dragonflies being the classic example of this). Others may be aquatic as adults, but will fly from pond to pond as the mood takes them (as is the case with water beetles). The second problem with insects as aquarium residents is that the smaller forms are invariably eaten by fish, as is the case with mosquito larvae, whereas the larger forms are often remarkably adept at catching and eating small fish, as with dragonfly larvae. All in all, while insects are fun animals to observe in a pond, they are of no real use in the aquarium.
Aquarists are typically offered two types of freshwater invertebrates, crustaceans and molluscs. On the whole freshwater invertebrates are undemanding and hardy animals. But this doesn't mean they are necessarily easy to keep. Firstly, not all the invertebrates offered for sale are compatible with tropical freshwater fish. In many cases the species offered require brackish water, coldwater temperatures, or dry land to walk about on. Secondly, the feeding habits of many invertebrates are unacceptable in many aquaria. Some species eat live plants, others will catch and eat fish, and yet others are filter feeders that require a diet appropriate to their needs. Finally, some invertebrates can multiply their numbers rapidly in aquarium conditions, rapidly becoming a pest rather than a pet.
Pet, pest, or waste of money?
Almost always, impulse buying of invertebrates for the freshwater aquarium to be a bad idea. While the marine side of the hobby has developed to the point where most of the invertebrates sold are more or less compatible with one another and amenable to life in the average reef tank, almost none of the invertebrates sold to freshwater aquarists are truly perfect residents for the average community tank. In other words, unless you have thoroughly researched the invertebrate in question beforehand and know how to care for it, don't buy it!
Molluscs - snails, clams, and mussels
Snails are unquestionably the invertebrates that are most easily maintained in ordinary freshwater aquaria and can be divided up into two camps, those introduced deliberately as novelties and those that come in as hitchhikers on aquarium plants.
Of the species deliberately kept in aquaria, apple snails, Pomacea bridgesi and Pomacea canaliculata, and Colombian ramshorn snails, Marisa cornuarietis, are the two most common. Although their shells have completely different shapes, these two species are very closely related. Apple snails are large and globose, typically about the size and shape of a golf ball, and the varieties sold in aquarium shops have yellow or golden-brown shells. Colombian ramshorn snails are not quite so big and have flat shells about the size of a large coin and are delicately marked with dark brown and yellow stripes. In terms of care these are very similar, requiring tropical rather than coldwater conditions and neutral to moderately hard water. Filtration is important, and both types of snail will die quickly if kept in poor conditions. Dead snails as big as these two species pollute the aquarium very rapidly. They are air-breathers, and must have be able to poke their breathing tube above the waterline or they will drown. On the plus side, the snails are easy to feed, and will eat a variety of green foods (including aquarium plants) together with catfish pellets and algae wafers. They do not breed all that readily, and the eggs are always placed in a pink-coloured mass above the water line (typically somewhere on the aquarium hood) where they can be removed as required. If they do breed, the baby snails can be reared on algae and vegetarian flake foods, and are usually easy enough to pass on to aquarium shops or other hobbyists. On the downside, these large snails can be incredibly destructive in planted aquaria. They do not always work well with fish either; nippy fish such as certain barbs and tetras can harass them to the point of death by persistently nibbling on their delicate antennae and breathing tubes.
Increasingly widely traded are nerites, a primarily marine group of snails with a fair number of brackish water and freshwater species. In their favour is the fact that most species appear to eat nothing other than algae and don't harm plants. None breeds readily in the aquarium, so population control isn't an issue. Most species have globose, shiny, often delicately patterned shells but a few are distinctly weird and bear odd little spines and flanges, adding to their novelty value. The chief problems with nerites are that they can be difficult to identify and that they are relatively demanding in terms of aquarium care. Identification is important because brackish water species are sometimes sold as freshwater snails, but will not do well under such conditions. All nerites need hard, alkaline water that is well filtered and contains lots of oxygen.
Pond snails are snails that live in still or slowly flowing waters and have evolved to withstand poor water quality and low levels of oxygen. Most are able to breathe air and many can 'hibernate' if conditions get bad, for example if the pond dries up. The classic pond snail are the ramshorn snails, Planorbis spp. These snails have very flat shells and come in a variety of colours, including brown, yellow, and red. Most are relatively small, about an inch across at most. Ramshorn snails are great opportunists and will eat almost anything, including aquarium plants and fish eggs. They breed readily in aquaria, producing clumps of jelly-like egg masses from which tiny but otherwise perfectly formed baby ramshorn snails emerge. Tadpole snails Physa spp. are perhaps more of a nuisance because they are smaller and o more difficult to find and breed much more quickly than ramshorn snails. While ramshorn snails are sometimes purchased as novelties for the aquarium, tadpole snails usually get in as stowaways on batches of aquarium plants or with live food. The Malayan livebearing snail Melanoides tuberculata, also known as the Malaysian trumpet snail (MTS) is a snail that divides the hobby. Some aquarists see them as a boon, others as a pest. On the one hand, they are totally harmless as far as plants go. They are excellent scavengers and keen burrowers, and in the planted aquarium do an excellent job of keeping the substrate clean and oxygenated, fulfilling the same sort of role as earthworms do on land. However, they breed rapidly, and any waste organic material in an aquarium not removed by the aquarist will be quickly turned into Malayan livebearing snails. Their strong shells also make them less easily eaten by snail-eating fish, and some aquarists debate whether it is even safe to allow fish to try and eat them, suggesting that the shells could damage the mouthparts of the fish in question. As is often the case, both sides of the divide make valid points, and as useful as these snails can be, it is important not to let their numbers get out of hand.
Bivalves (clams and mussels) are easily dispensed with as far as their usefulness in freshwater aquaria goes: don't bother. Putting aside the fact that many of the species sold as freshwater mussels come from temperate zone waters and won't live long in a tropical tank anyway, none of the clams or mussels in the trade feeds on anything other than plankton. So unless you are prepared to use a turkey baster or pipette to squeeze in portions of filter-feeder food (obtainable from stores catering to marine aquarists), any clam or mussel placed in a freshwater tank will die. Admittedly, it might take weeks for them to die, but die they will, through simple starvation. Contrary to what some retailers might suggest, clams and mussels cannot simply survive on the algae floating about the water, unless your aquarium water is so algae-laden it is the colour of pea soup! Nor can they 'scavenger' in the not clearly identified sort of way other retailers might suggest. Clams and mussels lack mouthparts and cannot chew up food or graze on algae. A happy clam is one that has dig itself into the substrate, so even in the best case scenario these animals don't put on much of a show, and should such a beast die hidden away in the sand, its decomposing body can create a nasty pocket of anaerobic decay as well as an extra load on the biological filter.
Crustaceans - crayfish, crabs, and shrimps
Freshwater crustaceans of various kinds are now regularly traded. These can be divided into five basic types: algae-eating shrimps, fan shrimps, long-arm shrimps, crayfish, and crabs. The algae-eating shrimps are exemplified by the Amano shrimp Caridina multidentata. They are mostly small (typically around an inch in length or less) and completely peaceful, doing best when kept in groups and maintained in quiet aquaria with small fishes like neon tetras and harlequin rasboras. Most are excellent algae eaters, though they will also peck away at bloodworms and other small, meaty foods. Large fish will eat these little shrimps, so choose tankmates with care. Fan shrimps are species of Atya (the big blue variety) and Atyopsis (the smaller red-brown varieties) from Africa and tropical Asia. They are fairly large (around two to three inches) and robust in build, but despite their size they are harmless animals that can work well in community tanks. In the wild they feed on plankton, and will certainly appreciate fin, particulate food, like crushed flake or liquid fry food, if provided, but they will also use their fans to collect algae and eat suitable foods from the substrate such as catfish pellets. Fan shrimps are territorial but otherwise peaceful, and in a large enough tank it is possible to keep multiple specimens. Their size makes them a better choice for use with standard community tank species like angels and gouramis than the small algae shrimps. Long-arm shrimps are species of Macrobrachium more normally valued as food than as pets. Nonetheless, various types get siphoned into the hobby and offered to aquarists. Some species are very large (over 12 inches) and all are active omnivores that will anything they can catch. Of marginal value in aquaria, they may possibly be safe with armoured, non-predatory catfish like plecs or dorads but are otherwise not recommended for anything other than their own aquarium. Territorial and aggressive, these shrimps will fight with one another and like to dig.
Crayfish look like small lobsters and behave in much the same way, being reclusive, nocturnal omnivores with a fondness of soft aquatic plants and carrion and a predilection for digging burrows and uprooting plants. They will catch and eat small fish if they can, and though frequently kept in community tanks they are not really trustworthy in this regard. Crabs are similar, but as well as being destructive and potentially predatory, they are also amphibious, and will spend the entire time trying to escape if kept in a normal aquarium. Crabs should be kept only in vivaria where they can move about on land and underwater. The most commonly traded species are the red-claw crab Perisesarma bidens, the Nigerian rainbow crab Cardiosoma armatum, and various fiddler crabs, Uca spp. All three are amphibious to some degree, the rainbow crab being almost entirely terrestrial in the wild. Fiddler crabs are deposit feeders and can be kept safely with fish; the other two crabs are distinctly predatory and will catch and eat small fish. Red-claw and fiddler crabs also happen to be brackish water, not freshwater, species.
Invertebrates are generally low maintenance in terms of healthcare provided water quality and water chemistry are optimal. Apart from the pond snail, all invertebrates are intolerant of polluted water or water with high levels of nitrate, and although some can breathe air (such as crabs and apple snails) most cannot and will quickly die in overstocked tanks with stagnant water.
A prime issue for aquarists keeping these animals is avoiding the use of copper-based medications. Most invertebrates, but especially crustaceans and apple snails, are notoriously intolerant of copper compounds of the types included in many popular fishkeeping medicines. Before treating a tank containing these animals, confirm with the retailer or manufacturer that the medication is copper-free and safe for use with invertebrates.
Crustaceans suffer from a further complicating factor in that they periodically moult. The biology behind this process is complex, and depends at least in part on diet and water chemistry, and under the wrong conditions they cannot moult and will die. Snails and clams generally grow steadily and without problems if they are receiving enough food, but they will suffer from pitting if their shells cannot be maintained properly, and once the shell is damaged it cannot be repaired. Though this is unsightly, it rarely causes the mollusc in question any serious problems, but it is a warning sign that the conditions in the aquarium are not optimal. On the whole, crustaceans and molluscs are most reliably maintained in hard, alkaline water.
The dietary needs of freshwater invertebrates are often overlooked, on the basis that they are essentially 'scavengers' in some way. Filter-feeding species will need appropriate particulate matter provided on a regular basis (ideally a small amount each day) and algae-eaters will need alternative food once the aquarium is cleaned of green algae. On the other hand, many snails and some crayfish will eat aquarium plants. Crabs, crayfish, and long-arm shrimps can and will catch surprisingly large and active fish (especially at night). Most snails will eat fish eggs, making them a definite no-no in the breeding tank. On the other hand, most large fish will eat smaller invertebrates. Loaches enjoy small snails, and pufferfish are very adept at eating larger snails, shrimps, and crayfish. Immediately after moulting, even otherwise durable shrimps are very vulnerable to damage, and can lose a leg or antenna to overly curious but normally harmless tankmates.
All this actually underlines a key difference between freshwater and saltwater aquaria. Marine aquarists routinely think of their livestock as an ecosystem, with fish eating invertebrates which eat algae. The algae and many of the smaller invertebrates at least multiply readily, creating a food chain of sorts that supplements the diet of the marine fish. Freshwater aquarists almost never think this way. Trying to slot one or two invertebrates into a freshwater tank is trying to force an accommodation between predator and prey that wouldn't exist in nature and isn't expected in a marine aquarium. The result is all too often that one or the other gets eaten, or, in the absence of its normal predators, multiplies wildly, as is the case with the smaller snails. End result: most of the time, invertebrates are best left out of the freshwater aquarium unless kept in their own space and on their terms.