Ok firstly I want everyone to know this thread is just my opinion on the two products, and which suites my tank best. Also remember the two products are not of the same spec (the Tunze is much more powerful in terms of l/h, so for an accurate comparison one should compare the Tunze 6025 with the Hydor Koralia 1 or 2)
Products in question: Tunze Nanostream 6025 (2500l/h) & Hydor Koralia Nano (1000l/h). I found the tunze pumped an incredibly strong flow in the 24G JBJ Nano Cube, so much so that my frogspawn and a few other softies didn't like it to much. The pump says its from 40 - 200l aquariums, I just found it gave an "washing machine" effect in my 24G, however in a larger aquarium this piece of kit is fantastic!
So I wanted a more gentle, dispersed flow. So I bought 2 Hydor Koralia Nano's (1000l/h) - much softer flow, but with the 2 I felt like I could get more dispersed flow. It appears MUCH gentler than the tunze, as these pumps are rated for a smaller aquarium and give a much wider flow.
Placement of the Koralias in my nano
I know now there are 2 pumps vs 1, and some people hate the look of equipment in a tank, but I don't mind the look and my midas blenny loves perching on the powerheads
Video of the flow: (The noise is from my chiller, removed the front to clean it!)
My clowns swimming in the flow:
Both are fantastic products, if I had a larger aquarium the Tunze would be great (or I could look at the Koralia 1 or 2), however in my 24G JBJ Nano I think the smaller pumps are better suited. Once again this is just my opinion... take it or leave it