I have 180 gallon cycled tank. 7 goldfish and one died a week ago. It had pine coned then lethargic then died. Now a second fish is lethargic and starting to pine cone. Water is ph 7.8, temp 78, ammonia 0, nitrite 0, nitrate 10 ppm. I do weekly water changes of 20%. I use prime. I feed soilent green gel food only. Unless they seem sick then I feed peas. I am not sure what to do or why they are all getting sick it seems slowly. I understand dropsy is internal condition. Are they spreading to each other? How can I cute it before they all die?