the hydors seem to have hude reliabilty problems imo, i had 2 koralia 1's both have died. i ended up buying a cheap powerhead from china via ebay and have to say it outperforms the koralias and at £25 im not gonna be as upset when or if it breaks as i was when my koralias died at £65 a pop. so go figure.
ive also had a tunze whilst albeit too powerfull for my tank it was well built and very reliable.
I curently have two Hydor Koralia 4's and am switching to a Vortech MP40 propeller pump Here you can also watch a movie of it Here and Here . This pump is the best one out there. There are many different settings that you can set it to. Well worth its weight in gold.
I have found that the Hydor Koralia gets stuck a lot. Not very impressed with it. Also, the little red arm that holds the head unit to the suction cup has broken in half on both of the units I have.
Couldn't wait to get in to work today to respond to this posting. I hate my Koralia!!!! What a coincidence, I spent almost 1 hour last night taking my Koralia 4 apart because it was making a loud humming noise and was seizing up. Upon taking it apart, which was not easy, the back broke out and is now sticking out. I managed to get the thing put together but I am not happy with it at all!!! I guess you get what you pay for. My advice, stay away from the Koralia. Yes, I do agree, they produce a very nice water motion and movement in the tank but as far a longevity, they suck!!!! Plan on replacing within a year.