I had goldfish (feeders) in my system and all seemed to be well. I added other goldfish and 4 Koi about 2 days later, and all was well for 5-6 days. The tank is 100 gallons. Then all the fish got a bit sluggish and are now dying off. I only have 1 Koi and 3 goldfish left. The water levels seem to be OK. Could I have introduced a virus into the system? If so, how do I get rid of it with food grade treatments. The plants still look to be thriving with no signs of stress. Any advice would be appreciated.