Pictures don't help much. Atleast not for me. With so many different parameters that can affect growth rates how big is the ray now? Possible to get a measurement if it ever cruises up the glass?
With all the little ones I have 6" doesn't sound to far off for a 5 month old. Also taking into consideration it looked pretty tiny from the original picture.
from the second picture it looks like is has a really fat tail and it also does look like it has grown..
my best advice is to get rid of that gravel asap...
good looking ray.