Also, consider using Purigen as chemical media. We recommend only using genuine Eheim spare parts in their range of Filters.
EHEIM 2260 CASE PRO
Fits the Pro 4+ 250, 350, 600 External Filters. I personally like it because I dont have to fill the filter baskets to the brim and this means improved flow. Eheim Pro 4+ Filter Pad Set 2617710 Triple Pack.
Stuff like Sera Siporax is reputedly better in terms of bio capacity/volume but it is generally considered that most externals have more capacity for bio filtration than would ever be necessary. I would say something like Ehfi mech pro and a layer of sponge for the mechanical, then whatever bio media you choose. As far as media for the new filter goes, the world is your Oyster. The lower your bioload the quicker you can do this. I hook up my water change hose directly to the eheim tap which makes water changes dead easy. This way I can replace the eheim taps if they fail. I have a 2260 I am going to use but will need another few filters. I would run them both for a month, then take increasing amounts of the media out of the Juwel over the next month. More a case of the seal failing or connection fails but I guess thats what insurance is for. I can understand why you said that but I can appreciate why he wants to run them both. Isnt 1/2 inch equal to 13mm Is the 16/22mm tubing that you want to use also PVC or is it just the Eheim plastic tubing If it is, then just use any plastic with a 16mm ID even if the wall thickness is thin and you can then slip the Eheim tube over it by running hot water over the end to make it more. Not necessarily that practical since a Juwel internal filter works mainly with square sponges which dont really relate well to using in the new filter.