A 90 gallon tank stocked with suitable (for the tank size) Malawi or Tanganykian cichlids? The two best choices for a spiney eel for that tank would be a Tanganykian elipsifer of plagiostoma spiney eel. The plagiostoma is the smaller of the two, maxes out around 8", and will eat the eggs of any substrate-spawning cichlids in the tank. The elipsifer grows a bit larger, a 75-90 is the minimum suitable tank size for an adult, and they're fry eaters.
I had a pair of plagiostoma's in a 29 for a couple years until I lost them during a power outage. They spawned a number of times in season, but the male always ate the eggs soon afterward. They did very well on a diet of frozen bloodworms, occasionally frozen brine shrimp for variety.