Fahakas are tempermental. It might work it, might not. A 20" cat is pretty big though, but at the very least you can expect the fahaka to pick at the poor guys fins. I've always wondered if it would work better to house Fahaka's with other nocturnal predators...that way the fahaka can't sneak up on them in their sleep during the night. I don't know though, if that giraffe cat is expensive and you are attached I wouldn't do it. Even if it doesn't happen now that fahaka is going to grow and I'm sure an 18" fahaka could easily take out a 20" fish if it suddenly decided it didn't feel like sharing the tank any longer.