Before anything else: make sure the bottom of the tank you're looking to drill is not tempered glass!
If it is, it'll just break on you when you try to drill it.
Will 1/2" diameter tube allow enough water out to keep up with the water being pumped in through another 1/2" diameter tube?
The question you really want answered is will a gravity-powered drain .5" in diameter be able to move as much water as your PUMP
pushes back into the tank through whatever
size input pipe.
There are a couple of ways to approach that, the simplest would be to buy an adjustable flow pump or put a valve on the return line and simply turn the inflow down to a flow the drain can handle. However, that might not be the best way to go.
For a more complex answer, I recommend you have a look at this site
The other thing you're likely to run into is that a drain like you're describing is susceptible to being quite noisy -- think of how a bathtub drain sounds. That webpage has some information along those lines as well, but its suggestions aren't going to be directly adaptable to what you're describing. The simplest answer to that one is going to be a larger diameter drain tube. How large will depend on how much flow you want to drain through it. This
might be an acceptable option. You could prowly manage it with clear tubing, if you're clever/creative/persistent enough.
What you're describing can be done, but it's likely to take some fiddlin' with. Since you are planning ahead, some experiments with plastic buckets, or the like, might be a desirable idea. I'd hate for you to get into dealing with the glass one and then try to adjust that
after the fact.