the wider the better. I have heard that they can grow up to 31" across. Rt now Ive had my Motoros for 3+ years and my female is now around 21" and my male is around 16" across.
Also, just my opinion, sand is better than gravel. Gravel is hard on a rays stomach area. Might think about switching it out to sand.
Also, just my opinion, sand is better than gravel. Gravel is hard on a rays stomach area. Might think about switching it out to sand.