PROBLEM TO BE SOLVED: To provide a water tank that can secure floatability and safety of plankton by making it easy to generate a moderate circulation water flow, make it easy to appreciate the plankton, and make survival times of the plankton long.SOLUTION: A water tank 1 includes a water flow generation part 11 disposed adjacently to a first wall plate 4, a third wall plate 6, and a fourth wall plate 7 along a bottom plate 3 for generating a bottom side water flow FL toward a second wall plate 5 along the bottom plate 3, a discharged waster part 20 disposed on a second wall plate 5 side with respect to the water flow generation part 11 along the bottom plate 3 for generating a discharged water flow FE that intersects with the bottom side water flow FL and is discharged to the reverse side of the bottom plate 3, and a first inclined part 8 disposed adjacently to the top face 11s of the water flow generation part 11, the third wall plate 6, the fourth wall plate 7, and the first wall plate 4 for generating a return water flow FR from a descending water flow FD that flows from the waster surface WF.SELECTED DRAWING: Figure 2【課題】緩やかな循環水流を生成しやすくして浮遊生物の浮遊性および安全性を確保し、浮遊生物を観賞しやすく、浮遊生物の生存期間を長期化しやすい水槽を提供する。【解決手段】水槽１は、第１壁板４、第３壁板６、および第４壁板７に隣接して底板３に沿って配置され、底板３に沿って第２壁板５に向う底側水流ＦＬを生じる水流生成部１１と、底板３に沿って水流生成部１１に対して第２壁板５の側に配置され、底側水流ＦＬと交差して底板３の裏側へ排出される排出水流ＦＥを生じる排水部２０と、水流生成部１１の上面１１ｓ、第３壁板６、第４壁板７、および第１壁板４に隣接して配置され、水面ＷＦの方から流れてくる下降水流ＦＤから回帰水流ＦＲを生じる第１傾斜部８と、を備える。【選択図】図２
