mata matas don't get any bigger then softshells.. infact softshells are fish eaters, and if you are talking a florida softy they get massive... and will actively hunt even larger fish, where mata matas tend to only eat small fish.. they will lie in wait and "vaccum" up small fish to eat.... and are imo pretty boreing turtles. Any turtle mixed with fish can = eaten fish. plain and simple. rarely does it work long-term. Turtles are also very dirty animals so make sure you accomidate filtration and water changes to match. You can also easily lose a small turtle to your fish eating it.
people do it with varying degrees of success... look up threads here about mixing turtles and fish.. 90% end up with eaten fish or eaten turtles. If you do decide to do it do your research on the species of turtle you want to keep and it's individual needs, before tossing it in. BTW most people w/ FRT (fly river turtles) report fish loses from the turtle and it's rare that a FRT doesn't attack it's tank-mates at some point.
