1/4” will more then likely be fine but it depends on the size of the plumbing. For anything less then 1.5” I would use 1/4”. For 1.5” I would use 3/8” but 1/4” would also work. I should have been more clear & called it by the right name but what I was calling airline fitting is the same thing as the John guest fitting. John guest is just a name brand, they have different manufactures. They are actually labeled as 1/4” O.D by 1/2” Nptf push connect fittings. The 1/4” would be the push connect side for the airline & the 1/2” would be the male threads. The trick is getting the treaded part of the fitting airtight. A bunch of people including myself just tapped the pvc cap on top of the tee & screwed the fitting in. It would be easier to just use a pvc bushing in the top of the tee. So say if u have 1.5” plumbing u would use a 1.5” by 1/2” bushing. Make sure the 1/2” side is female threads & the 1.5” side will slide straight into the top of the tee. Then the John guest would screw straight into that.
RO tubing will work but it can really be any polypropylene tubing, so nothing fancy. If u have some to tubing just use that, if not Home Depot sells poly tubing for really cheap
This is the fitting for the airline
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