i am a user for the Hydra filter, and you can check my tank thread in the forum .
From my own finding, the hydra work but it do required a bigger rating for marine tank use , and debris will affect the function as well maybe due to the chocking up of the carbon.
Till date, i cant find any reading from any existing water test kit that can show result before and after use, but the water clarity and fishes showing more lively is a good testimonial that it work for some way.