Overweighted at beginning of dive but underweighted at end in shallows

[inquis]

I have found that I get as much bottom time (or more) from an AL63 as an AL80, since I am not having to account for and fight the positive bouyancy. of the 80

You may very well get more bottom time out of the AL63, but the fact it stays negative isn't the reason. Similarly, an AL80 having positive buoyancy *in isolation* is fairly meaningless since it's rigidly attached to the BCD (and effectively so to the diver as well). By your logic, you could say the same thing about a wetsuit since it's always positive, but that is equally not applicable. It's the buoyancy of the entire system that matters.

 

[tursiops]

Aluminum 80s are notorious for getting light as they empty, and will float up at the tail end, as well. For my own tanks I prefer steel 80s which stay negative. If renting, an AL63 will stay relatively neutral. Over the years, I have found that I get as much bottom time (or more) from an AL63 as an AL80, since I am not having to account for and fight the positive bouyancy. of the 80

This is an often misunderstood point.
You don't get buoyant at the end of dive because you are using an AL tank. You get buoyant because you have used up much of the gas in the tank (no matter the material the tank is made of) so you are now (typically) 5 lbs lighter at the end of the dive than your were at the beginning.
If you are fighting buoyancy at the end of the dive, you need to carry more lead.
A steel tank weighs more in the water than does an AL tank, so the need to carry more lead is less likely....but being overweighted at the beginning of the dive is also more likely, especially if you are using a steel tank AND are carrying lead.

 

[Belzelbub]

Similarly, an AL80 having positive buoyancy *in isolation* is fairly meaningless since it's rigidly attached to the BCD (and effectively so to the diver as well).

Or, at least it should be. Near empty AL80s tend to float bottom up a bit. Some of the BCs with a single strap mounted just under the shoulder may have to base of the tank moving around a bit. But, I agree, a properly secured tank with dual straps isn’t moving much.

 

[Bob DBF]

If you are fighting buoyancy at the end of the dive, you need to carry more lead.
A steel tank weighs more in the water than does an AL tank, so the need to carry more lead is less likely....

One issue I have found with the AL tank because it is butt light, is if one is horizontal, it floats the back of a jacket BC up, and any air in the BC will now be higher in the water column and expand exacerbating the buoyancy problem. I would insure the BC is empty before adding too much weight. That problem doesn't arise with a steel tank because it stays negative, or neutral.

 

[tursiops]

One issue I have found with the AL tank because it is butt light, is if one is horizontal, it floats the back of a jacket BC up, and any air in the BC will now be higher in the water column and expand exacerbating the buoyancy problem. I would insure the BC is empty before adding too much weight. That problem doesn't arise with a steel tank because it stays negative, or neutral.

Seems like you are describing a problem due to an overly large BCD that doesn't fit and/or is not tightly cinched.
I'm also finding it hard to image that a couple of inches of vertical displacement in the vicinity of 15 feet has very much effect....Boyle's Law applies, but insignificantly. 3 inches out of 15 ft means a volume change of about 1/60, or about 1.6%....like carrying 12 lbs of weigh and suddenly carrying just 11.8 lbs. Letting a little air out of the lungs more than compensates for that difference in buoyancy.