and only having 3 fish in the tank. Is this even possible. With the sliders as shown if you want less than 7 fish you have to make a choice about what those fish are going to be (i.e. you can't have a random choice of less than 7 fish). You would need a separate box to say how many fish you wanted to show.
You're right. With my method, it's difficult and non-intuitive to specify less than 7 fish if ANY of them are random.
Well, if we were going to start from scratch, I see Prolific's idea of dragging fish or a Question (?) mark into 7 or 10 slots (since the 3D background version will have 9-10 fish slots) and then having a separate panel "Customize Random" similar to my prototype above.
Also, invertebrates could be integrated into this design.
Let me do a new prototype.