Cleaning a 15 gallon fuel tank

[eterry]

My nephew thought the twin 6 gallon fuel tanks were steps getting out of the boat. He broke them both.

I found a guy online who had a boat with inboard 15 or 20 gallon tank. I bought it, but it has lots of trash and debris in it.

I've flushed it several times but am still getting trash in my fuel line.

How do you pro's clean a fuel tank with no large opening?

 

[eterry]

I figured a shop owner had dealt with this and had an idea. Small gravel, sand, marbles have been suggested.