I have 2 preformed ponds: 175 gallon and 210 gallon. I don't know as they are deep enough to keep fish in throughout winter. I live in lower michigan. Last year, I brought my fish inside and they did pretty good. I would like to leave them outside if possible. Maybe use some sort of heater to keep ponds thawed? Also last year, I left my water lilies in the pond (which froze over). The lilies did come back this year. I brought in all my reeds and grasses. I was told to cut them down and wrap them in wet newspaper and keep the paper wet. They did survive. I stored them in my garage. Can I do this for my lilies? If anyone has any better ideas tho on how to winterize my 2 ponds, I would appreciate it.