I agree, 20 is just too small. you'll need a 55g a 75g or something a little larger. Size isn't everything though, you'll also need excellent filtration to deal with the waste
The point here is that bigger is better even if it's a temporary quarantine tank due to it's special needs, pristine clean water, no amonia, nitrites, or high levels of nitrates. Rays are more sensitive when it comes to these water parameters than most fish. Look at it this way, you have the 300 for it,..... you wouldn't want to skimp out during the quarantine process & potentially do more harm than good. Personally I would get a used 75 gallon or so with some 10x turnover rate filtration -- using cycled media.