depends on the dimension of the 300vs210. If the 210 is 7x2x2 and the 300 is 8x2x30", you really arent allowing too much more floor space for a ray. I moved my female motoro out of my 300g 3ft wide(it was sold yesterday) into my friends 220 (6x2), until theyre shipped next weekend, and the female is exactly as long as the tank is wide. It takes the whole 2ft for her to move. Skip the ray if you want to keep one long term, or look for the harder to find hystrix