i hav 1 red finned shark or ruby shark as you call it in one of my community tanks. he chases anything that comes to near but he never harms them just chases them away from his spot. he is a great character. i brought him when i had a large one in there already. he nearly died from being attacked by the other. but mannaged to recover by hiding in a plant in one corner. then my larger one passed away and the other one began to venture out and become more bold. he is now 5-6 inches.