Fuel tank pick up and antisiphon valve

[tsubaki]

They are normally made of stainless, brass and aluminum.
Normally they unscrew with an open end wrench.
At the top or bottom of the pickup tube there should be a filter screen (probably stainless), check it for cleaning.
If you can see the bolt heads on the sending unit, try to tell if they are stainless, could be the problem. Or some jackass dropped something in the tank, like a bolt or nut.
Also don't rule out the filter rusting on the inside, cut it open and see.