All things considered, there are no good tankmates for the SUT.
Tankmates for Surinam Underwater Toads
Surinam Underwater Toads (SUT’s – Pipa pipa) are fascinating animals to keep by themselves in a species tank, but the question is often asked what other fish or animals can be kept with them as tankmates. The correct answer to that question is “not much.” The problem is twofold. To start with, SUTs have a huge mouth, and they will try and stick absolutely anything that might be food into the mouth. I have always fed mine feeder goldfish, and it is fascinating to watch their capture style. They sit there completely motionless, and when a goldfish swims anywhere near the mouth, the SUT literally “explodes”, instantly sucking into its mouth anything within six inches of it. It spits out the gravel and other stuff that is not food, and the goldfish is lunch. So, anything under say 6″ in length will probably end up as a snack.
The problem with larger fish is that the skin of the SUT, while fairly tough, has all kinds of little flaps and other appendages that are used as “lures” to bring in unsuspecting fish. The corners of the mouth have what look like little worms, and when a fish picks on them it is over in a split second. Larger fish that cannot be considered food will be able to pick on the SUT without being swallowed. This is very detrimental to the toad. All things considered, there are no good tankmates for SUTs.
There are two other underwater frogs that, in addition to the SUT, make very good animals for an aquarium – with a couple of provisos.
The Dwarf African Frog (Hymenochirus boettgeri) is an excellent addition to any tank where none of the fishes are large enough to eat these little guys. They only get to be an inch or so at maturity, will do fine on regular fish foods, and are small enough not to bother any fish.
The African Clawed Frog (Xenopus laevis) is an entirely different animal. Most commonly called “Xenopus” or simply “clawed frogs”, they get to be about 5″ or so at maturity, and they get to that size by eating anything they can fit into their mouths. Very often an unsuspecting hobbyist will bring home a cute little “dwarf” frog, have it turn out to be Xenopus, and notice as the frog gets larger that some fish are missing from the tank. These frogs can eat fish up to the size of an adult swordtail or molly.
Even if the local fish store where you buy these frogs is very good and professional, sometimes their suppliers are not, and therefore “dwarf” frogs can also include small clawed frogs. There are two ways to determine which species you are going to buy. First, look at the front legs of the frog. If there are claws at the ends of the finger, the little guy you are looking at is a Xenopus, and will grow to about 5″ or so, and will get there by eating anything it can. Also, if the frogs being offered are albinos (yellow to pink coloration) they are Xenopus. True dwarf frogs, the ones that stay dwarfs, are not available in an albino form.