I just got mine going recently... I am waiting for one of those nano-diffusers I bought on ebay to hopefully spread the bubbles out a bit. Now, it jsut comes out as one big bubble and floats to the top where it bursts. I am not sure if this is even doing anything for the tank.
PS... I make beer also, so I know a little about yeast. The yeast actually never dies, it hibernates once it runs out of food. There are breweries in Belgium which are still using yeasts that are hundreds of years old. Theoretically, you could keep reusing your yeast, but it probably is not practical in a DIY set-up where the new yeast cost next to nothing anyway.