Hmm... not really "for no reason". Every book and magazine article about
Mastacembelus spp. makes the point that these fish become increasingly territorial and aggressive as they age. This contrasts with the spiny eels of the genus
Macrognathus, which tend to be much more tolerant, and indeed often do best in small groups.
In terms of tankmates, the
Mastacembelus armatus-species group tends to be tolerant of midwater and upper-level fish. If you're keeping them in freshwater, then mid-sized barbs, rainbowfish, silver dollars, etc will all work well. The main thing to remember is adults get fairly large; at least 50 cm, and potentially up to 90 cm. So you do need to choose tankmates with that in mind. Things like swordtails may have length in their favour but their narrow bodies could put them at risk. I've seen (unfortunately) even half-grown
Mastacembelus armatus gobble up platies without any problems at all. So choose deep-bodied fish that aren't at risk. They don't bite their prey, so won't eat things they can't swallow whole; rather, they subdue and eat prey small enough to swallow whole. Mostly it's worms, but fish are on the menu for most spiny eels, given the chance.
Some of the
Mastacembelus armatus-species group naturally occur in brackish water. As such, you could keep them with large sailfin mollies.
Cheers, Neale
I once had a tyre track and I thought he was great. he coexisted with a fire eel amongst others for a few months but then for some reason the fire eel just started attcking him and killed him which I was gutted about so I wouldn't mix those two again