Seachem bottle stoppers come in two sizes and allow you to seamlessly connect dosing pump tubes to any Seachem or Aquavitro bottles ranging from 50 ml to 1 liter and many other common supplement bottles (24 mm - 28 mm) in the market. They are ideal for use with Aquavitro Synthesis dosing tubes. They fit tube sizes up to OD 0.192 inches / 4.88 mm. Small bottle stoppers: fit 50 ml - 500 ml (24 mm) Medium bottle stoppers: fit 1 liter (28 mm)