The Spiny Eels, the Mastacembelids. This is the most appropriate group of freshwater eels for aquarium use. 

    The first clue as to these fishes not being anquilliform comes in their common appellation, "spiny" eels; as the true eels lack spines in their fins. They do have decidedly "eel-like" elongated bodies, lack pelvic fins and their continuous dorsal and anal fins come together with or without a caudal/tail fin.

    Differentiating them from the true eels is a pectoral arch ligament linking the fin process to the vertebral column, as well as the aforementioned series of 9-42 isolated spines in front of the dorsal, and two or three more spines afore their anal fins.

    All told there are two subfamilies, four genera,  and sixty seven species of Mastacembelids. Subfamily Mastacembelinae show a distinct tail fin and have prominent rostral appendages (see above image). Consisting of two genera (Macrognathus, Mastacembelus) of about a dozen species each, the mastacembelines are confined to Southern Asia from Iran to Korea.

    The subfamily Afromastacembelinae, found in Africa, through Syria, have caudal fins which are confluent with their dorsal and anal fins. Two genera, Caecomastacembelus (= Afromastacembelus) and Aethiomastacembelus have 24 and 18 species respectively. This subfamily is rarely offered in the U.S. Its care is similar to the more Asian subfamily, but be aware that some members are pronouncedly more territorial and predatory. 

Species of Use/Availability to Aquarists:

About Miscellaneous Spiny Eels: 

Just what sort of category is this anyway? There are several, from the Middle English meaning "many" species of Mastacembelids that are offered as "misc.". These tend to be brownish, some with tire-trackish markings, e.g. Mastacembelus circumcintus, under a foot, and have the same general care requirements as other spiny eels. 

Maximum Sizes:

    Though most spiny eels stay small (under a foot), many of the popular species can attain three feet, even in captivity. About the only thing that slows their growth is jumping out.

Selection, General to Specific:

    Spiny eels are rarely ambivalent imports. They're either all live and vibrant, or falling-apart dead. Once they reach the retailing end of distribution this trend can and does continue. What to look/select for:

1) Time in captivity: 

The longer the better, as long as the eels are feeding. At wholesale outfits I've worked at we've had the most "luck" with a protocol of immediately pouring the whole box of fish through a very large square net, placing them in all-new water of neutral pH, 75 F., and adding Methylene blue till the water is very dark. Salt is added daily to the semi-open systems to curtail infection and extra airstones added for enhanced gaseous exchange, dissolved oxygen, and distribution. A few times, when it was obvious all were going to be lost on arrival otherwise, we would inject pure oxygen gas into the water of their systems. I have witnessed this technique literally bring them back from death.

2) Apparent Sores:

Should any of the spiny eels in the system exhibit red areas, particularly near fin origins, or white, blotchy markings, leave all of them. They are likely to be soon dead. 

3) Behavior:

Does it make a difference how "outgoing" a specimen is? Sure. You want yours to be viewable,  maybe even hand-trained. Avoid spaced-out specimens that swim erratically at the surface or bottom. 

4) Index of Fitness:

How "buff" is that spiny eel? These fishes can go on hunger strikes for a few to several weeks; obviously of benefit in situations where food is unavailable seasonally. Still, you want a "full-bodied" specimen. Healthy ones are definitely girthy.

5) Size at Purchase:

Related to demonstrated feeding, this is a note to beware of purchasing large specimens. Big wild-collected Fire Eels in particular are notorious for refusing food on importation. Better to purchase sub-adults and grow them up yourself, or seek ones that have been traded in from good homes.

6) On Netting:

This is an area fraught with danger for these fishes. They are literally "slippery as eels", and given to wriggling up and out of specimen containers and nets right on to the floor. Further, they tend to jab hands and bags with their short by stout fin spines. So, do use deep, soft nets and prepare a double-bag of depth with water in it, in anticipation of catching Mastacembelids.

    By the way, don't despair if you find your intended target has burrowed into the substrate. Utilize a large-enough diameter mesh netting and scoop the eel and gravel up together. Voila! Gravel drops out, fish is caught!

Environmental Conditions

Habitat

    Though they can/will adapt to bright, open settings, spiny eels are naturally reclusive, preferring darkened corners. Many species are burrowers, spending their days in mud or unconsolidated bottom material with only their "nose" sticking out. The best specimens I have observed were kept in large, well-covered systems replete with live plants, non-sharp, fine substrate, caves, flowerpots or tubes for hiding, and subdued lighting. 

Physical/Chemical

    These fishes are not overly sensitive to any given set of water conditions but appreciate regularity. This is best brought about through frequent partial water changes. Temperatures should be tropical but not range over 80 F. Elevated ranges tend to bring on disease, and increased metabolism while decreasing oxygen concentration.

Filtration

    With such meat-eating fishes it's obvious you will need vigorous, complete circulation and adequate mechanical filtration. What may not be so obvious is these animals need for high oxygen tension. Though they're bottom dwellers and may seem sedentary in their ways, the spiny eels used in the hobby typically come from fast moving waters. You should supply just as agitated/aerated water, through redundant fluid moving pumps and/or mechanical aerating bubblers. 

Behavior:

Territoriality

    To a number, these fishes get along with their own and related species. I have seen many numbers of them of all sizes thrust together in public aquariums and wholesale operations without incident. Likewise they are tolerant of all but the smallest of fishes, though they will gladly consume crustaceans, worms of smaller than mouth size. 

Introduction/Acclimation

    This THE most critically important interval in your spiny eels life. Most individuals are lost during the first few days of moving. The following notes mainly apply to large dealers and wholesalers who initially handle these animals (and lose the most), but are worth considering by the home hobbyist for the good they can do them in preventing losses. 

1) On arrival and during transport, pay close attention to these fishes breathing rates. Make spiny eels a high priority for aerating, water change and transfer to their holding/display space.

2) Do treat them prophylactically for scrapes and very probable infections. They are all wild-collected and tend to come in "rough" physically from handling, scraping against each other (a note, some species are collected as ornamentals and food fish by dry-pumping basins in low water season... stressful) and being held/transported in bacteria-laden water. Old-timey Methylene blue and Acriflavine added to their water are of value, as is the diligent addition of non-iodized salt (about one teaspoon per gallon). Wholesale operators should add these daily in semi-open or open systems over the course of a few days.

3) For home hobbyists, should your spiny eel show signs of "breaking down" (red marks at fin origins, white loose skin markings, unusual diffident behavior), I encourage you to remove it to a quarantine/treatment tank and treat it as above, post haste. 

Predator/Prey Relations

    Other large fishes have been known to eat spiny eels, spines and all; and as mentioned, they may eat very small tankmates given the chance. The byword here is appropriate. The right tankmates make for no problems. You want to select appropriate ones. Ones who don't hassle the spiny eels or eat up all their incoming foods... and ones not so little as to become midnight snacks. 

Reproduction, Sexual Differentiation

    There are a few foreign and domestic accounts of spawning in captivity (see below). Females are larger, fuller with an apparent genital papilla. After vigorous animated swimming together, clear eggs numbering in the several hundreds of a little more than 1mm diameter are scattered, generally in live plant material near the surface. They hatch out in about three days, are about 6 mm long and feed on small live foods. 

Locomotion

    I'd like to remind you of this groups predilection for burrowing. Arrange your rock work appropriately so it won't fall on your spiny eel/s; and do give consideration to having suitable gravel, and/or adequate "tube" spaces for their comfort.

Foods/Feeding/Nutrition

      All meaty foods are taken easily, with familiarity. Crustaceans, mollusks, insect larvae, particularly live worms of all sorts are acceptable. An important note regarding foods is that they must be bite size or smaller; these fishes do not chew off pieces of larger food items.

    In the wild these fishes feed principally at night; in captivity they grow to recognize and respond to their feedings at any time.

Disease

    This is a sore topic with this family of fishes; having only small embedded scales and being otherwise soft-bodied does not help; they are susceptible to physical traumas, bacterial and parasitic infection. If you think your fishes are coming down with ich, look to your spiny eel/s to show it first. 

    Spiny eels don't respond well to toxic dye and metal medications. As with tetras, catfishes, and other small scaled fishes, careful half-doses, adding salt and diligent water quality checks and changes are appropriate. "An ounce of prevention...".

Close:

   The spiny eels of the family Mastacembelidae are worthy aquarium subjects. Paramount to their successful husbandry is securing healthy specimens, providing adequate covers to prevent their leaving, meaty foods of appropriate size, and decor to hide in/amongst.

    Should you decide on a Mastacembelid member, be sure to pick one out that fits your tank size and tankmate make-up... and keep that lid on!

Bibliography/Further Reading:

http://aquariacentral.com/fishinfo/brackish/eel.shtml

Axelrod, Herbert R, Warren E. Burgess, Neal Pronek & Jerry G. Walls. 1990. Atlas of Aquarium Fishes. Reference Book v. 2 Freshwater Fishes. T.F.H. Publications, Inc. NJ. 1055 pp.

Baensch, Hans A. & Rudiger Riehl. 1993. Aquarium Atlas v.2. MERGUS, Germany 1212pp.

Brittan, Martin R. 1998. A taxonomic look at mastacembelid spiny eels. TFH 7/98.

Carroll, Kevin. 2000. Gentle giants with a twist. TFH 6/2000.

Fenner, Bob 1998. Eels, eels, eels. TFH 7/98

Finley, Lee. 1983. Mastacembelus brichardi (Poll). A blind spiny eel from the rapids of the lower Zaire basin. FAMA 1/83.

Kocetov, Sergei M. 1992. Breeding spiny eels. TFH 1/82, FAMA 4/92.

Nelson, Joseph S. 1994. Fishes of the World, 3d ed. John Wiley & Sons, NY. 600pp.

Stokes, P. 1965. Breeding the spiny eel, Mastacembelus panculus. TFH 7/65.

Walker, Braz. 1969. The tire track spiny eel. The Aquarium 5/69.

Walker, Braz. 1976. The spiny eels. Aquarium News Journal 5:2/76.

Wolff, Ulrich. 1977. Spiny eels- interesting in appearance and behavior. Aquarium Digest Intl. 4, 1977 #18.