Yes like everyone said that tank is small. Get a male motoro pup 4-5" in size and you may get 18months if he is housed in the 90 alone. Good thing is, you could easily sell him when he is bigger, or, if you cant save a little money in 18 months do buy a bigger tank, you dont need a ray. They cost a TON to house with all the water changes and FOOD! haha
I went out of town for four days. My rays are all 8-10 and I fed them as usual before I left and had my fiancee feed them once during that time span. Anyhow, they decided to try and eat my HUGE 15" fire eel. The eel is still alive but pretty banged up. Rays can be some evil bastages when they are hungry!