style.css

 .tank {
    position: relative;
    width: 200px;
    height: 300px;
    border: 3px solid black;
    border-bottom-left-radius: 20px;
    border-bottom-right-radius: 20px;
    border-top-left-radius: 1px;
    border-top-right-radius: 1px;    
  }
  .liquid {
    position: absolute;
    bottom: 0;
    width: 100%;
    background-color: blue;
    opacity: 0.2;
    border-bottom-left-radius: 20px;
    border-bottom-right-radius: 20px;
  }
  .sq {
    position: absolute;
    height: 150px;
    width: 150px;
    top : 200px;
    left : 200px;
    background: red;
  }