The firefish is a goby, their natural defense is to escape to their "bolt" hole. I'm going to assume it doesn't have one. It would be dug into a deep sand bed, or a long, narrow cave. Assuming it was unable to dig a burrow, You can create a plastic cave using PVC piping or something similar, needs to be narrow. If too complex, hold the one of the fish in a floating box or grilled tupperware, like a colander. This is a "time out" spot GL