I agree with Tammy's well stated comments with one exception - while you can arch your back to maintain trim, it forces you to use the large muscles in your trunk and the large muscle groups use a commensurately large amount of O2 and then put out about 80% of that volume in CO2. Consequently, that approahc leads to a higher than needed SAC rate or, if you try to artifically lower your SAC, to elevated CO2 levels which exacerbate issues with narcosis and potential oxygen toxicity.
It is well worth the time and effort to work on your trim so that you can hover motionless in the water and stay fairly close to horizontal without having to use back muscles to lever your self into that position.
As noted above, the tank can be moved for and aft on the BC to alter the trim. An issue here is that an AL 80 tends to get lighter in the tail toward the end of a dive, so if you trim for neutral at the start of a dive, you will be slightly feet high at the end, but that's not all bad.
Light (positively buoyant or neutral fins) versus heavy (negatively buoyant) fins can also make a difference in trim, especially as the moment arm is quite long.
Positioning of weights can help, and for a recreational diver a trim weight on the cam band or an ankle weight around the tank valve can help solve a head up trim issue. Redistribution of needed weight is ok, but adding weight is always a bad idea as it aggravates the buoyancy control issues.
Overweighting requires you to carry extra gas in the BC and that larger volume of gas also changes volume more with slight changes in depth, especially in shallow water. So more weight equals more gas which equals more change in buoyancy and the need for more frequent adjustments.
Ideally, you should have just enough weight to stay neutral at 15' at the end of the dive with 500 psi in the tank and no gas at all in the wing. That allows you to hold a safety stop without effort and if you are in a wetsuit it will leave you slightly positive on the surface, even without adding gas to the BC. With an AL 80 that also means you are no more than about 4 pounds negative at the surface with a full tank, so overweigthing should not be an issue.
Your lung volume will cause a buoyancy swing of 4-5 pounds as well, so you will never truly be "neutral" but will instead shift from positive to negative as you inhale and exhale. The good news is that inertia is your friend here as a 1-2 pound shift in buoyancy takes a while to have an impact on your total mass.
The bad news is that this also means there is some lag when making adjustments to your buoyancy and this is where the practice. practice, practice thing comes into play. With lots of practice, very small additions or dumps from the wing, and careful attention to detail, you will develop a feel that will allow you to note the impact of breathing on bouyancy, anticipate buoyancy changes with changes in depth and add just enough or release just enough gas from the BC to achieve and maintain neutral buoyancy concurrent with an efficiant breathing cycle. Eventually you get to the point you can use inhalation and exhalation timing to initiate and stop descents with very minor releases or additons during and at the bottom or top of a depth change to hold the depth when you reach it - and do it without really having to think about it. But it takes practice and you have to be aware of and pay attention to the big picture until you develop a feel for it.
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And not to start a BC versus BP wing argument, but in my opinion, precise buoyancy control is much easier in a backplate and wing because you are more more solidly connected to your buoyancy control source and can feel what's happening a lot better.
