One of the water level sensors was improperly soldered. Posting here for reference.
Note that in the two following images, one can see some amount of electrical connectivity between the ground and power pins. Any amount of solder or wire that bridges those two contacts will short power to ground. We don't want any extra electrical shorts, so this is always a potential problem, but in this case, it could completely fry the entire system.
For future soldering,
[ ] Check to ensure no solder bridges two or more pins. For 0.1" pitch connections like this, you should be able to see unsoldered space between pins with the naked eye (no microscope needed, but check with scope if you wish).
[ ] Trim extra wire using flat cutters to make a tidier connection and to reduce odds of nearby wires touching each other (and shorting the connection.
One of the water level sensors was improperly soldered. Posting here for reference.
Note that in the two following images, one can see some amount of electrical connectivity between the ground and power pins. Any amount of solder or wire that bridges those two contacts will short power to ground. We don't want any extra electrical shorts, so this is always a potential problem, but in this case, it could completely fry the entire system.
For future soldering,