When to replace the filter?

Question
I bought a 5 gallon tank in August 07, it holds my one little Betta fish Spazz. He keeps getting fin and tail rot, I do 100% water changes every week. This didn't start happening until a month ago. So, I was wondering does this have to do with the filter? And how often are you suppose to replace the filter. I think its a bio whisper filter.

Answer
First thing is your defeating the reason for having a filter in the aquarium, the objective is to create a natural filtration system with a good bacteria base, when you do a 100% water change your causing your aquarium to start the nitrogen cycle all over again causing toxic levels of ammonia to develop causing your fish his illness, it normally takes 2 to 4 weeks for the aquarium to cycle correctly, once this is done you should only do 10 to 15% water changes per week.  What you can do to try to save your Betta is add some aquarium salt to the aquarium in the amount of 1 tsp per gallon water, raise the tank temperature to about 82 and keep it that way for about 4 days, then do a 20% water change and do not add any additional salt, in 4 more days do another 20% water change and start to lower the tank temperature back down to 76F - 78F and keep it in that temperature range, in another 4 days do another 20% water change and then go to standard weekly water changes of 10 to 15% for maintenance, keep about 1 tsp of aquarium salt per 5 gallons always present in the aquarium, this will keep your Bettas immune system up and help control any parasite or bacteria outbreaks.