Alrighty.
Here's a new attempt at a prototype. I'm trying to focus on usability and something that looks good and is easy to figure out, based on all the comments here so far.
I don't know if Jim has reconsidered the Invertebrates limit. I put 10 fish slots and 4 invertebrate limits.
To make things easy for users, I think dragging an invertebrate to a fish box should automatically move it to the inverts box and vice versa. No point in hitting the user over the head with a beep or dialog box when the Aquarium can sort it out in such a way that satisfies 95% (the other 5% will be upset that the last slot on either the fish or inverts got replaced with whatever they dragged).
It might make sense to remove the Advanced tab and make it a button that appears on the Fish & Invertebrates panel at the bottom that only appears if you have at least one random fish selected.
This is pretty close to Prolific's idea of how fish selection would be done, except I have an Advanced tab for Random fish. To have multiple random slots, drag multiple question marks to as many slots as you wish, replacing fish if needed.
Note that 1.0 and future versions have fish set to all 7 or 9-10 slots set to random, so it might be nice to have the Advanced tab available without a second step. I don't know if new users will understand going to the Fish & Invertebrates window for the first time and seeing 10 question marks.
Maybe we need a better icon to represent Random.
Click here to go back to the first page and see the new Settings Panel B.
Oh and...
