We used to hang beer on a fish stringer off the side of the boat, when we needed to free one up, we freed it up
(so they were in there for a very long time) :? Never had one do bad things.
I've also found cans at the bottom of the lake that had been there for quite some time, no rust,
AFAIK, rust requires oxygen, so it will rust at a slower rate underwater, then being pulled out..
Aluminum does not rust, but will dissolve/degrade. mind you, the timeframe for that to happen will likely out live your fish..
I have no scientific background so take it as you will, aluminum should not have any ill effects to water chemistry as long as you keep to regular water changes, if you neglect water changes then aluminum could build up in the tank, chemi-pure which removes most heavy metals might be an option aswell..
Most paint on beer cans is baked on, as such it will be on there long after the can is dissolving, it's a type of enamal, and I doubt it would cause any problems (and since it lasts for such a long time, it's doubtful it leeches anything.)
If you wanted to use an empty can, then you would have to pour boiling water through it.. perhaps bleach etc... then I'ld fill it 7/8 the way with sand and use Aquarium safe silicone to top it up, this will stop fish from going in and slicing themselves up...
Just my thoughts, and I might be wrong, but it's what makes sense from the little understanding I have of real world exp. (oh, and technically,
PH would make a huge difference in the dissolving rate of the aluminum, the more acidic, the faster it dissolves, again, from my understanding...)