I tried and I got my husband to try. He put a lot of muscle to it and said it would break if he pulled any harder. I did get it moving around a little. I could actually see the tank moving said:
OK, this is probably a stupid question, but how is the pipe connected at the bottom of the tank? Is it glued into a bulkhead? If so, then pulling wouldn't do any good, but unscrewing the bulkhead from underneath should make it (and all the water/fish in the overflow) come right out. Even if it's just pressed into the bulkhead but stuck, you should be able to unscrew it from underneath, right?
I can't really see how it's connected at the bottom due to all the sediment, so I could be way off here...