Give one quick spray of Armorall into a gallon of fuel. Armorall is a vinyl cleaner for car dashboards so you'll find it at any auto parts shop.

In my experience, many times the cause of the bubbles is a leak in the tube inside the tank where the silicon tubing meets the brass tube. If this is the case then you usually do NOT see bubbles when the tank is full, but the bubbles appear when the tank gets half full.

Armorall has been used for more years than I can remember here in Oz. The active ingredient is the silicon it contains and what it does is reduce the surface tension to such a level that bubbles can't form. Sort of exactly the opposite to dish washing liquid
All that's needed is ONE quick spray into a gallon of fuel, there's no point in using any more than that. But just so you can see what it does, before you spray it in give the can a good shake to make it foam then give it one squirt. The foam will magically disappear in an instant.
BTW, there are some people who insist that the silicon will blow plugs or coat the platinum so they stop working. Not true.
I've never heard of anyone using it in petrol but I can't see how it could do any harm. Maybe I'll try it in my lawn mower and see what happens

Jimbo,
First you need to determine if you really have air bubbles or foaming. Armor-All cures foaming, fixing leaks cures bubbles.
Good luck! Sometimes leaks are hard to find. Don't exclude the possibility of a hole in the brass tubing also.