(As an aside, "Quantum Physics for Dummies" is surprisingly readable and concise)
TIPS: If your fishtank lid is especially cheap, the dry ice may crack it. Also, make sure you place the tank in a room that you can make completely dark - trying to move the thing to a darker room destroys the trails :)
* Eating a banana: 0.1 µSv
* Background daily dose: 10 µSv
The calculation is probably more complicated, but as a back of the envelope estimation I expect that a banana increase the background radiation in a 1%: So it will be difficult to measure with a cheap equipment just counting the number of detections.
[If your device can measure the energy of the particles, perhaps you can see some peak?]
[If you log all the event for a day, with and without a banana, perhaps the 1% difference is statistically significant.]
I found this youtube video showing a similar setup in action: