Camera & Lense Recommendation

[TheReefGeek]

I appreciate the help, thank you so much.

This may be a dum question, but why do you need two flashes?

The IS on the 17-85 will help with hand-held aquarium pictures won't it?
For nature shots, I mean walking/hiking, and I see landscapes and animals I want to shoot, won't the IS be a help there?

The price difference between the 70-300 IS and the 70-200 F4 L is very little, which would you recommend then, the 70-200 even though there is no IS? My other lense would likely be the 17-85 IS.

Maybe I am missunderstanding what IS really does, can you explain?

Do you have any experience using extension tubes for macro photography without an actualy macro lense?