I'm not familiar with the Whisper power filters or the media they take, but when you say that you change the cartridges every 2-3 weeks, does that mean that there is all brand new media in the filter every time you change them out? If so, you could be recycling your tank each time you change these. Generally, filter media should be rinsed in old tank water and replaced to maintain the beneficial bacteria. Could probably use a larger filter on that tank also, sized for the twice the volume.