Marine aquarium

[S1mon.]

Any one on here run a marine aquarium?
I've got a 6x2x2 tank at the moment the i keep Ciclids in, but i have someone to buy the whole set up and I've been looking into marine. loving the colours, and all the live corals that go with it.
Would be looking at the same size tank, but confused as to what filter system to go for, either external filters or a sump tank.
Can some one point me to the pro's & cons on each? And any other do's or don'ts
Thanks.

[S1mon.]

Thanks for the advise, and links. some good reading.
book ordered, managed to get it under £8 on ebay