will you like to use any impeller-based filter with a sand substrate where turtles are concerned.
Especially with active swimmers like redears and a burrower like a softshell, you'll get maximal exploitation of the sand-stirring niche. It would likely be a better idea to house the softshell separately from the redears, especially considering that the full size of either species would permit only one animal to comfortably live in a 55 gallon tank.
Large and small grained sand alike will ruin the impeller of virtually any filter, and since impeller-driven filters tend to be the only kind strong enough to clean turtle tanks, sand is generally not a viable option.
You might be able to use a trickle filter setup if there is a very fine prefilter before the pump, but worthwhile pumps alone are often more expensive than many canister filters.
Considering the ease with which gravel is cleaned over sand, that alone could guide one to chose never to use sand except in specially-arranged setups with softshell species that need such a substrate (spiney and Florida softshells will normally do fine with a fine-grained gravel). Good luck.
