How does the Ocean Free Hydra filter work?

No other filter does what a Hydra does, so it’s hard to compare it to anything. In a nutshell, these filters prevent new tank syndrome by converting ammonia into nitrogen, just like a mature biological filter but without the bacterial action. Having seen them do amazing things that were hard to explain, we tested them: https://www.fishkeeper.co.uk/stories/hydra-findings

As we’re also sceptical about things that sound too good to be true, there’s a few things that they don’t do.

• These filters won’t cope with additions of large quantities of ammonia and are designed to remove small quantities as they’re produced, by a normal stocking of fish. The large Hydra Stream units can handle larger waste loading and we’ve seen these in action on pond systems with great results.
• A Hydra generally reduces nitrate by preventing the accumulation of new nitrate. When placed in a high-nitrate environment, levels may not drop rapidly unless other measures such as water changes are carried out. Supported by the use of low nitrate water sources, such as RO, a Hydra can keep levels low. These units will only reduce nitrates below the level of the source water as part of a mixed approach to nitrate removals, such as the use of resins or denitrification.
• A Hydra won’t prevent filter bacteria from colonising your aquarium. Although they reduce the available nutrient source for them and hence limit the populations, heterotrophic bacteria will be present and should the need arise, they can quickly multiply to pick up the slack.