IME the best spiney eel for a tank smaller than 75 gallons is the Tanganykian plagiostoma. I had a pair of them in a 29 for a couple years, lost them both to a bacterial bloom that lowered the O2 too much. They spawned multiple times, but the male always ate the eggs soon afterward. They're hard to find, and almost always overpriced when they are available, but they max out around 8-9" and don't burrow in the substrate like most species.
Mine ate mostly frozen bloodworms, which gave them great color along with conditioning them for spawning in season (November-March.)