Im sorry but if you are worried about money and buying a tank and don't know anything about the nitrogen cycle or how to cycle a tank or practically anything, then you dont need to keep a ray, and possibly not a reg. fish. You need to read read read! study and research all the stickies on this forum, not just the ray stickies, but all about filtration and tank sizes... honestly a motoro will be OK in a 180gal for about 1.5 yr max. then you will need a bigger tank again. IF your really hard on for a ray, then save the money and buy the tank 1st, take the time to research all the criteria needed to having a stable filtration system, and triple up on your filtration. i have a 280 gal tank, and run enough bio filtration for a 500gal aquarium maybe more... and you WANT turnover, which is how many time an hour the tank water runs through the filter.... more is better, i run a 4800gph pump in my filter to keep the water moving quickly.
but seriously man.... a tank that is barely 50gals is the WORST tank for a ray. you would have to do a water change once a day mayabe more...
please do yourself or should i say, SAVE yourself the heartache and the waste of money and do NOT buy another ray until you have done all of the above