Mine is about 3-3.5 feet deep with koi. My koi are about 6-7 years old and about 2 feet . Havent lost any in a few years with this set up. Anywhere in that range you can have the fish winter over. I would look into getting a really good filter system . Thats where people cheap out and they always come back to the store with issues. We sell the liner from firestone, it has a 20+ warrentee on it . We carry a underliner as well that you place under the liner to help protect roots from puncturing holes.