Tunze Nano DOC vs AquaC Remora
 

Which is better?

As always the next question is what is it for? Otherwise you could be comparing apples to oranges.

In theory Tunze is a much better product, however I have seen a lot of DOC skimmers with early pump failure. At least with the AquaC you can replace the pump with a standard MaxiJet. I'm assuming you are referring to the in tank skimmers and not the in sump models? The AquaC Remora is a good skimmer but you must buy the overflow kit or you are completely defeating the purpose of a skimmer and the theories behind it.