I used 5000 LBs rated concrete...it was about $5 a bag. We used about 200 bags....but some of those were not used for the pond. I would guess we used 165 bags for the pond....... helpful hint ...If you can dig down and have some of the pond below grade and keep your walls under 3' tall your pond will still have the depth you want with out losing any strength. We went 2' below grade and 3 above. We did add a 4 foot window....really adds to the enjoyment of the pond/tank.