That is so very true! When my 110 gallon aquarium was freshwater, I had a few pet stores that were scared to sell the fish I asked for. They claimed they weren't compatible from water temp to aggressiveness. I had a Large Pleco, bala/irredescent sharks, all kinds of different cichlids, severum, oscars, jack dempsey, variety of eel, mud cats, channel cats, a huge red belly pacu, a wild brim, butterfly fish, angels, black moores, koi and blood parrots. All of my fish were hand fed so that may have made a difference. The only one that ate feeder fish was the mudcat. The weakest of the bunch were my oscars and parrots. Would have added a stingray and puffer, but decided to try out saltwater.