You must have a pump AND a filter. Don’t let anyone tell you different! It doesn’t matter if your pond is a whiskey barrel on your patio or a preformed 4000 gallon pond. Our rule of thumb is that EVERY gallon of water needs to be turned over (pumped through a filter(s)*) each hour. However, this usually only applies to ponds up to 500 gallons. For most ponds over 500 gallons each gallon of water should be turned over no less than once every four hours. Please note most books will recommend turning the total water volume over each hour. If you reach a balanced eco-system and maintain it such a large turnover isn’t necessary.
There are several reasons for cycling the water this often. One of most important reasons is for filtration. Whether a mechanical or biological filter (or both) are used, the bacterial cannot be filtered out without passing through a filter.
Circulation is also very important in smaller ponds due to the water temperature. For fish and plants to remain healthy, the water temperature should maintain a constant healthy level. People often forget this when placing a 120 gallon preformed pond in full sun. Utilizing pond plants would also reduce this problem.
* In conjunction with a mechanical and/or biological filter.