Hans: Nigeria, really ?
It's most likely sludge from old fuel. My T2 sat for 9-13 years and the plastic of the tank is intact but the fuel itself turned to tar.
Many different ways of fixing it, but basically the best thing would be to remove them, clean them with mineral spirits and small pebbles and then rinse them out very well. If you have access to an air compressor, it will make drying that much quicker.