An apparatus for generating and mixing gas bubbles into an aqueous solution, the apparatus comprising: a structure defining an interior fluid-flow chamber extending along a longitudinal axis between an input port at a liquid input end and an output port at a liquid output end, wherein the structure is characterized by: a gas injection portion located upstream from the liquid output end, the gas injection portion defining a gas injection lumen and a first region of the interior fluid-flow chamber, the first region of the interior fluid-flow chamber comprising a plurality of side fluid-path lumens (922a, 922b) that extend in a downstream direction alongside a first part of the gas injection lumen, wherein the first part of the gas injection lumen and the plurality of side fluid-path lumens merge with a downstream fluid-path lumen of the first region, and the first part of the gas injection lumen is closer to the longitudinal axis than any of the plurality of side fluid-path lumens; and a mixing vane portion extending in the downstream direction from the gas injection portion and defining a second region of the interior fluid-flow chamber, wherein the first part of the gas injection lumen extends to a downstream opening in fluid communication with the downstream fluid-path lumen, and is aligned to inject gas through the downstream opening and into the downstream fluid-path lumen in a direction downstream, characterized in that the structure is a unitary, single-piece structure, and in that the downstream opening is void of any structure that would impede the flow of gas. a structure defining an interior fluid-flow chamber extending along a longitudinal axis between an input port at a liquid input end and an output port at a liquid output end, wherein the structure is characterized by: a gas injection portion located upstream from the liquid output end, the gas injection portion defining a gas injection lumen and a first region of the interior fluid-flow chamber, the first region of the interior fluid-flow chamber comprising a plurality of side fluid-path lumens (922a, 922b) that extend in a downstream direction alongside a first part of the gas injection lumen, wherein the first part of the gas injection lumen and the plurality of side fluid-path lumens merge with a downstream fluid-path lumen of the first region, and the first part of the gas injection lumen is closer to the longitudinal axis than any of the plurality of side fluid-path lumens; and a mixing vane portion extending in the downstream direction from the gas injection portion and defining a second region of the interior fluid-flow chamber, a gas injection portion located upstream from the liquid output end, the gas injection portion defining a gas injection lumen and a first region of the interior fluid-flow chamber, the first region of the interior fluid-flow chamber comprising a plurality of side fluid-path lumens (922a, 922b) that extend in a downstream direction alongside a first part of the gas injection lumen, wherein the first part of the gas injection lumen and the plurality of side fluid-path lumens merge with a downstream fluid-path lumen of the first region, and the first part of the gas injection lumen is closer to the longitudinal axis than any of the plurality of side fluid-path lumens; and a mixing vane portion extending in the downstream direction from the gas injection portion and defining a second region of the interior fluid-flow chamber, wherein the first part of the gas injection lumen extends to a downstream opening in fluid communication with the downstream fluid-path lumen, and is aligned to inject gas through the downstream opening and into the downstream fluid-path lumen in a direction downstream, characterized in that the structure is a unitary, single-piece structure, and in that the downstream opening is void of any structure that would impede the flow of gas.