I've cleaned three fuel tanks using the method outlined below on compact tractors and riding lawn mower to ensure they are clean. The riding mower was the worse as it had leaves and other crap in it (bugs, twigs, etc..). Besides miscellaneous debris in the diesel tractors, algae can also be a problem.
1. Drain fuel (If possible)
2. Remove fuel cell (if unable to drain earlier, agitate fuel before pouring out the filler inlet.)
3. Clean inside of tank using Castrol Royal Purple or equivalent cleaner. Shake around and let it set for a while.
4. Rinse with a hose including to flush from the fuel delivery port
5. Let dry (Warm sunny day helps)
6. Put a bit of fresh fuel in the tank, slosh around, drain and dispose of fuel. If there is any soap/water residue hopefully this helps remove before refilling.
Use fresh new fuel when refilling.
Yes, it is over kill. But I'm reassured that the tank is 100% clean.