They should be okay for the most part, but when the Kuhlis become very active at night, they may make an overzealous leap into the box filter and require some rescuing... You could tie pantyhose over the intake to prevent their "escape", but it should, usually, be pretty good. For adventurous Kuhlis who love to inspect every nook and cranny, I would suggest HOB or Canister filters, but a box filter should be okay.
Soop Nazi