I inherited a small hot tub with the house I bought. Basic dimensional analysis says it is 250-300 gallon and a "5 person" by comparison shopping. I know the guy just got non-working ones from Craigslist and refurbed them; it's a hobby of his. So it's old but it works. But there is no air injection in the Jets. There is a spot near the control panel with a blank plug where the air controller should be. Maybe he didn't want to bother but I thought it would be nice to add. What do I need to figure out so I can add the proper Venturi valve or whatever I need for passive air injection?