Most of the smaller spiny eels have circular spots on the tail/dorsal fin so that doesn't really help very much. And something like half a dozen species have been sold as "peacock spiny eels" at one time or another. Spiny eel nomenclature in the hobby is just appalling, and retailers take next to effort to identify their stock properly.
For the most part though all you need to do is figure out whether you have Macrognathus or Mastacembelus. Generally, Macrognathus are smaller, more sociable, and primarily invertebrate eaters. Mastacembelus tend to be bigger, much more territorial, and very predatory towards small fish. You can reasonably assume any Macrognathus will get to around 30 cm in the aquarium, while the common Mastacembelus in the trade are in the range 70-100 cm. The only common Mastacembelus in the trade are the fire eel, the tyre-track eels (plural), and the various African species. Everything else are species of Macrognathus from India and Southeast Asia. The smallest ones are about 15 cm, the biggest about 50 cm, but most in between.
Cheers, Neale
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