Considering the Rena XP2 or Eheim 2217 or 2075. Not ordered the tank yet (hoping to get 125 litre or 180 (wife says it's too big though...) or even finished the stand but got so many ideas! Too much painting and sanding going on at the moment. Is it the right move in planning to remove the Juwel internal straight away and go external?
No question, go external!
Things to bear in mind with the Eheim, there are no trays for easy maintenance in those models I don't think, the media needs to be bought separately and they are costly for what you get.
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I highly recommend the tetratec ex1200, for the money it is superb, there are 4 trays inside with all the media included (bio balls, ceramics, sponges, floss pad and carbon in bags). You'd want to put the carbon to one side as you want to keep it in case you have to remove any meds later on, not much point in using it straight away. The flow rate is good and it is very easy to maintain having a valve on each of the in and out hoses and a connector plate for the hoses so the whole hose assembly can be taken away easily to get inside.
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I've heard good things about the Rena too, although I can't remember which models...
Whatever you choice going external is a good choice.
You may want to consider an external heater too? I am using Hydor Eth heaters so all I have in the tank is in and out pipes
Also, I am not sure if you can get a Fluval Roma 125 instead of a Hagen Rio 125, but these have separate internal filter / heater so no messing with siliconed in bits and pieces, plus you'd have Fluval spare parts in case you'd need them then. Fluval and Hagen are the same make as far as I am aware, just slightly different in setup.