The larger ones are harder to keep. When there's less nutrients within the water, they die off. There's not a known process on how to keep them alive cause they still don't know exactly what they eat. The smaller ones that hitchhike on your LR usually do pretty well unless there's less nutrient within your water, then the number will decline eventually. I have a small one in my tank, now there's 3 of them and their dusters have grown bigger too! I didn't know they would grow? They're twice the size they were before. :happyfish