You are lucky you found it so quickly. A week isn't really enough time for "stuff" to petrify completely. Remember, even with the drain open, the tank is still a damp environment. If it had been 6 months later, you may not have been so lucky.
Bobbo and Lin
2017 F-150 XLT 4x4 SuperCab w/Max Tow Package 3.5l EcoBoost V6
2017 Airstream Flying Cloud 23FB