I purchased my cat fountain more than three years ago. I have never had a problem with the carbon filters until this summer.  I weekly clean the fountain, parts, foam filter and rinse and inspect the carbon filter.  The quality of these filters changed in the last six months.  My new carbon filter is always rinsed on both sides until the water runs clear.  Now each week I have had to replace these carbon filters that are supposed to last 2-4 weeks and because it leaks I sometimes have to replace the foam filter to because they turn black.  I'm spending in one month almost the same price for filters that I paid for the fountain. Is this what I should expect from now on from the filters?  If so it will be time for a different fountain.