Drain the tank and replace strainer.

Then use Startron enzyme treatment and it will sort the problem out in around a few tankfulls of fuel.

I use it in my home made biodiesel and never have any trouble.
Startron eats up the bugs and then they just go through engine and get burnt and exit the tail pipe, other biocides only kill the infection and then whats left blocks your filters.
Give it a go and save yourself a lot of work.