So I have a 300 gallon Rubbermaid stock pond that have three Cortez stingrays that are thriving. A zebra moray eel and a bamboo shark. Other fish as well that are stingray safe.. My total water volumne is 750 gallons.. I have tried to add other stingrays, blue spotted, diamond blue spotted, and a guitarfish. After like 2 weeks they are passing away. Are my 3 rays taking these guys out? Prob 6 months ago I tried adding another ray and the same thing. I am at wits end... Any ideas would help..