DIY Venturi

Hello Folks!

I've been trying to build my own DIY venturi valve out of 6/8" PVC tubing.
As far as I understand, where
the water inlet is, the hole will narrow down to about 2/8" dia in a very
short distance like 6/8" lgh. At the
bottle neck of 2/8" dia, it wll run around 3/8" straight before enlarging
back to 6/8" via a distance of 1.25".
An air tube is inserted into the 2/8" dia straight section.

I tested it in an unused fish tank filled with fresh water and the bubbles
are rather big. Not as fine as
those of ready made skimmer. Does anyone knows why?

Is there a difference in testing it in sal****er against freshwater?

Does anyone have any advice in making a venturi valve?

Thanks!

Regards,
Mark