What exactly is the function/need of the "floaters"?
The purpose of the floaters is to attach the main 8 sections of legs to the upper and lower frames. They float so that load is not transfered to the floor of your house or to the tank/ply that the tank sits on. All load is transfered to the 8 outer sections of the legs.
And do you think the 2x6s were necessary?
The original design states that you should be fine using 2 x 4s for tanks upto 4 feet and 2 x 6s are used for tanks up to 6 feet. Bottom line is I didn't need to do it, but I am glad I did.
After thinking a bit about this question some more, I have some relevant information why I would use 2 x 6s for this type of frame again and again. When I built the upper and lower frames, the lower frame was warped and twisted pretty bad. The upper frame was level and true. I was somewhat worried about this as I thought that the stand woudn't sit level or it would have wobble to it, etc. Since the upper frame was level and rock solid, I attached the legs to it first. Once I was done I attached the lower frame to the legs. Everything pulled together and the warping and twisting of the lower frame was brought in check.
The bottom line for me is that the 2 x 6s are level and true if you get a couple of good lengths. Had I used 2 x 4s for the upper frame instead, I think I would have some major issues with the stand being level and wobble free. Instead of having one frame with some issues, I would have two. If they had the issues in "the same direction" problems might be compounded.
I agree with the tendency to overbuild, but just thought that stand looks mighty heavy. Especially when you add in the 1/2" ply.
What do you think it will weigh when finished?
The stand isn't too bad yet. I can still handle it at this point. I can't really tell you how much it will weigh, but I know it is going to be heavy. I will try to get a good estimation once all of the other trim, doors, etc. are added.