Hi I have just joined this forum and came across your thread. I run an external filter on my tank. My friend has. Just done the same as you, going from an internal to an external. This is what he did. He took the media out of the internal and squeezed it into the external so some of the natural bacteria is now in the new one. He left the internal running along side the external for 2 weeks before taking the internal one out all together. I know I'm probably to late in replying but like I said I have only just joined, thought I would try to help encase you haven't sorted it.