It's very hard to order in a type of Mastacembelus Spiny Eel and for it to actually arrive as the species labelled (with the exception of Fire Eels,
M. armatus and
M. dayi)
I've never actually seen the correct fish arrive when M. frenatus has been ordered in. It's usually
M. cryptacanthus, an
M. armatus variant, a Macrognathus species or an undescribed fish.
Going by a small to mid-sized Mastacembelus growth rate, feeding daily a varied diet of chopped whitefish fillet, mussels, earthworm, insects and small soft carnivore pellets, should get you around
half an inch a month til 8-10 inches, then it'll be progressively slower.
Avoid foods which are constipatives such as prawn and shrimp (they also struggle to digest shrimp shells, causing blockages) and also get them off bloodworm or any other microplankton if you can help it.
Being in captivity, they have the opportunity to grow larger, faster and live longer and a regular varied diet rich in nutrition can aid that.
Good luck mate and has it arrived yet?