Confusing. The image is a species called Macrognathus aral, a freshwater/brackish water species that gets to about 50 cm or so. It is sometimes traded as the "one-stripe spiny eel" because of the distinctive olive band along the flank. It's a nice, fairly sociable species.
However, Mastacembelus erythrotaenia is a much bigger (~100 cm) freshwater species. It is an anti-social species that can be delicate in captivity. It's common name, fire eel, stems from its brilliant yellow/orange/red markings on its dark grey body.
Neither species is 30 cm!
There are some fairly small spiny eels, like Macrognathus pancalus (to 18 cm) but really no spiny eel is ideal for a tank as small as 15 gallons. Much better to stick with, say, kuhli loaches. There are various species including some quite chunky forms, and they are sociable, simple to keep, and easy to obtain.
Cheers, Neale