Tankmates for Rays
Stingrays are typically quiet fish and prefer to avoid conflict whenever possible, only using their spines for self-protection, so avoid purchasing any aggressive or territorial species as companions for your ray.
Although rays are relatively peaceful by nature, they are still predators, and they will attempt to pick off any fish small enough to fit in their mouths. Some smaller tankmates are easily ambushed at night while the stingrays are hunting, so be sure provide plenty of hiding spots so that they can sleep undisturbed.
In your search for good ray tankmates, your best bet would be to select larger, non-aggressive, less active fish that inhabit the upper levels of the tank. Below you will find general information on a number of fishes commonly kept with rays:
Medium-sized to larger characins
There are many good choices for tankmates in this group, including:
No pirahnas please!
Peaceful New World Cichlids
Rays get along fantastically with non-aggressive New World cichlids such as severums (Heros severus). Just be certain that the cichlids are large enough not to be considered food by your rays, and they aren't stealing your ray's food!
The bichir/ray combination is successful if the fishes involved are of similar size. The bichir must be large enough not to fit in the ray's mouth, but small enough that it won't attempt to chew on the ray's disc. Despite the fact that both types of fish are bottom dwellers, they make fantastic tankmates. Just be sure that you provide the bichir with plenty of places to hide and rest.
Above: One of the more widely available species, the senegal bichir, Polypterus senegalus.
Below: An armored or banded bichir, Polypterus delhezi.
Procede with Caution
Armored catfishes (a.k.a. "Plecos" or "Algae Eaters")
Ah, the trusty pleco… These fish may be a long-time aquarium staple, but these days, you don't have to settle for the dull-colored, garden variety "algae eater" that you remember from your third grade class aquarium or the waiting room at the doctor's office. There is a vast assortment of pleco species readily available at your LFS, and many of them make quite attractive additions to your fish collection.
The ray/pleco combination can work, but certain conditions must be met first. There have been reports of plecos latching onto the rays' discs with their sucker mouths and irritating both the ray and his or her skin. This is not as much of an issue in a large aquarium (≥1000 gallon) where the fish are not house in such close quarters, and the ray has room to get away if any attempts at disc-sucking are made. In addition, the catfishes must be large relative to the rays, and there must be plenty of places for the them to hide. The ideal pleco companions for larger rays are large royals (Panaque nigrolineatus) and sailfins (Pterygolichthys gibbiceps).
Arowanas (all species but jardinis)
Many people keep smaller gars of the Lepisosteus genus with their stingrays. The gars tend to remain towards the top of the tank, while rays will cruise the bottom, so they will stay out of each other's way most of the time. But if the two fish should meet, there could be some problems.
Rays are very active animals, and will often climb the glass or cruise mid-water when they are hungry. Gars can be skittish, and if frightened, will slam into the side of the tank and break their backs. Or they might lash out suddenly and injure the ray if they are being pestered. It can be an interesting combination, if you have the tank space. For more info on gars and other ancestral fishes, be sure to check out PrimitiveFish.com.
Aggressive cichlids - Africans, Peacock Bass, and Oscars
Try to avoid fishes like this that suck, regardless of whether or not you keep rays......
Discus & Angelfishes
These peaceful South American cichlids are often kept with rays because they prefer the same water parameters, and they tend to stick to the upper levels of the tank, out of the rays' way. However, they are quite sensitive and don't react well to the excessive stress caused by active rays climbing the glass and cruising about looking for a meal.
The discus/ray combination can make for a beautiful display in zoos and public aquariums, but in the home aquarium, where swimming space is at a premium, you're better off avoiding it.
South American and Australian Lungfish
South American lungfish (Lepidosiren paradoxa) and Australian lungfish (Neoceratodus forsteri) are the two most docile species of lungfish and seem to get along well with stingrays. These fish have powerful jaws, however, and one bite has the potential to cause serious damage to a ray or even kill it. Be very cautious with this combination.
Below: A South American lungfish swims with a group of P. reticulata and P. orbignyii.
Koi or Goldfish
Turtles and other tetrapods
Anything "bite sized"