The scene where the A-Team is in the tank and its falling from the air, they fire the canon and it stops from falling for a moment before falling again. Is this possible from a Physics point of view?
Let's do some sums. I can't find specs for the main gun, but a similar British Army 105mm light gun fires shells weighing 15.1kg at 709m/s. Using 1/2mv^2, that's 3.80MJ. Let's assume all of the energy from the firing went into the projectile when the tank was on solid ground, because that's the worst case - anything else would improve our chances of stopping the tank. Since the tank and projectile are now free-floating, half the force (and hence half the kinetic energy) goes to the tank.
The lightest M8 tank was 19.25 tons, which is 17,463kg. If the tank was falling at 14.7m/s (53km/h), one shell would stop it completely. From there on, doubling the speed needs four times the shells - so 29.5m/s needs four shells, 59m/s needs eight shells, and so on.
That's discounting acceleration from gravity between shots, of course. The main gun manages 12 rounds a minute, which is 5s between shots. If the tank is free-falling, that's 9.8m/s/s acceleration, which clocks up 49m/s between shots. So even if they weren't moving when they started firing, they couldn't keep the tank up. It seems unlikely that the single remaining chute (after the Reapers had shot the rest) would slow the tank enough that the gun could even keep up with gravity, never mind bring the speed down enough for a survivable impact on the lake.
Think of a large speaker cabinet. If you stand up a piece of paper a few feet away and put the volume of your stereo all the way up, the paper will be blown over. Much like Sonar, the compression waves move air toward an object.
Now, to stop a steel tank, those waves would have to be incredibly powerful. It's not likely that compression waves from a tank cannon could generate enough force to stop or even slow down an object that likely weighs a couple tons travelling at terminal velocity.
Tank guns have the technology to compensate the Recoil of the fired shoot.
This Wiki article is about recoil and mordern ways to compensate it.
And this article is about the Muzzle brake - also to minimize the recoil.
Both technics are straight against the fact that a fallen tank can be slowed down by shooting its gun. It would only start to spin around.
The only other thing I could think of that could (very unlikely) help them is the therorem of self lifting bodies.
But not only therefor the Tank must have gotten rid of the parachute, it also needed to get a bit more round on its edges.