A lot of the reptile sites claim that expanded coconut fiber mixed with play sand is the best substrate for russian tortoises. Our russian tortoise was given to us and his substrate was a larger reptile bark. Neither of the above is really appropriate. The best would be a smaller reptile bark mixed with expanded coconut fiber and play sand. 60 - 70 percent small reptile bark with 30 - 40 percent expanded coconut fiber and play sand (50/50 mix) should be the best.