Why you should always add a biocide to your diesel fuel (see photographs)!
The algal gloop formed a mat lying on the bottom of the tank. As there was a 3 cm gap below the fuel pickup to the flat bottom of the tank, the fuel siphoned out of the tank was clean. Removing the tank to clean it was not an easy job. Better not to let the bug grow in the first place.
The fuel was red diesel and the tank had been fully topped up before storage, as recommended. No water was visibly present in the tank.
I know from my experience with a garden tractor that this bio-gloop will block up a metal gauze filter very easily, let alone a finer filter. The engine would have stopped very quickly if the mat of biological material had broken up and been sucked into the fuel offtake pipe. On the tractor I have been adding a biocide (diesel additive) to every can of fuel that I buy and the problem has not reoccurred.
Even if difficult, you way wish to inspect the bottom of your fuel tank!