What kind of filter do i need for a 29 gallon saltwater aquarium. I have a 55 and am using a fluval 404.?

That should do it. Take it real slow with saltwater. Its a lot if work, a lot of money for the fish and can be very frustrating if you rush it and they die.  The fluval 404 is a good filter. Put a carbon bag in there too. Just go to a good tropical fish store and ask questions. Onyl pick the heartiest fish for the first 4 months.i would recommend a wet dry filter the provide the best  biological filtration there is.also add a protein skimmer.
   i would not add fish for 14 days then add a damsel,and then wait for another two weeks before adding any thing else that will start  your bio filter.