First off, never change your filter cartridges because you will loose a large portion of your bacteria. I had the same filter at one point but got rid of it for an upgrade. All filter manufactuers say "change your filter cartridge ever 4-6 weeks" They say this for 2 reasons, first they want you purchasing new cartridges every month, second these cartridges usually have carbon in them and the carbon needs to be replaced every 4-6 weeks. So what does this all mean to you.
Ok I will try to make this really really simple. You have a few options:
1. follow the instructions that came with your filter and replace every 4-6 weeks (not reccommended due to loss of bacteria)
2. keep the bio foam but replace the carbon packs every 4-6 weeks (better choice but still not optimal)
3. Keep the bio foam, and every 4-6 weeks open up the carbon pack (very easy to do with the Tetra whisper - just lift the green clips) empty the carbon in the garbage and replace the carbon only (make sure to rinse before adding back to the tank and rinse in old tank water to minimize bacteria loss) - better but still not the best choice.
4 - the best option is to open the carbon pack remove the carbon and replace it with some other kind of biological filtration such as ceramic peices, noodles etc. You really don't need carbon in your filter unless you are trying to filter out medications that were used in the tank. You can fill that space with better bio media (as mentioned above) which will vastly improve the surface area for bacteria to grow.
You have still one other option, but it really depends on budget and type of tank. I would suggest getting rid of the Tetra whisper filter and get a hang on the back (HOB) type filter such as Aquaclear or marineland filters just to name 2 brands but there are several good brands of HOB filters that will give you better biological filtration. Personally i would reccommend the Aquaclear filters, it has a media basket so you can use the media that comes with the filter or you can completely customize your media.
What you are looking for with media is somekind of course sponge to get the big bits, bio media such as ceramic peices or noodles to allow a greater surface area for the bacteria to grow on, and if room and you so choose a finer sponge or filter foam to "polish" the water (I find filte foam is best) this helps to collect any finer peices before it goes back to the tank.
Hope I didn't confuse you too much and it helps some. Good Luck.
