Tyre track eels are freshwater fish. Nearly all references I have read on them says that they would benefit from about a teaspoon of salt/gal of water, and they are listed in the pinned topic in the brackish forum.
Tyre track eels grow to be a large fish, exceeding 3 feet in length/~6 inch diameter. They should not be kept in any tank less than 180 gallons. It is best to put them in the largest tank possible that you can afford as they are growing, I am sure you understand this if you have ever been faced with trying to catch one
I wouldn't recommend housing the convict together with the eel, Tyre Tracks are opportunistic predators, even if the con is too large to fit into the eel's mouth, he may try anyway.
These eels, although piscavores, are very peaceful. You also want to be careful and not keep any agressive fish with a tyre track. They are nearly "scaleless"; their delicate skin can be easily damaged by other fish, sharp tank decorations/substrate, etc., leaving them especially vulnerable to fungal/bacterial infections.
