When I first built my reactor, I tried using an Aquaclear 301, but it wasn't
near strong enough and also required too much tubing in the tank. I now use
an extra Magnum 350 I had as the pump. This works out great. This gives my
Magnum new life. I can now run it all the time and use whatever media I
want to filter with and then the filtered water is injected with CO2. The
force of the water coming out of the filter is also slowed down some since
it goes through the reactor and doesn't create any additional current in the
tank.

Here are pictures of my reactor, based in whole off of several designs I saw
on the net.

http://www.leetsauce.net/~liquid/DIY-CO2/reactor/

Before I built this reactor, I was using the venturi port on a Aquaclear
201. That seemed to work ok, but I ended up with a bunch of tiny CO2
bubbles all in the tank and a bunch of them collected on the surface and
just rode the current. With the reactor, there are no more bubbles in the
tank and my CO2 levels have risen to where they should be, as well as my PH
falling from 7.5 to 7.0 over the course of about a week.

jesse