I recommend vinyl if you want flexible tubing, or either polyethylene (good), polypropylene (better), or FEP (best) if you want semi-rigid.
Avoid silicone if possible, it has the worst permeability rating of most materials.
"Standard airline tubing" is 1/8" inner diameter 1/4" outer diameter, but if you go with semi-rigid, you may need 3/16" inner diameter in order to get the tubing to fit your hose barbs & check valves.
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