Agree, a carbon sponge after a period of time will be simply acting like any other sponge, including harboring some of your beneficial bacteria. Thus it can simply be left in to continue to perform this biological filtration function (as opposed to chemical filtration, which is what carbon specializes in) and everything will be fine. Some day when you decide to replace it, you must take in to account that it may be performing some of the biological function and possibly replace only pieces of it one by one over time (or in some cases the you might decide the sponge only represents a smaller fraction of your overall biomedia and that the whole sponge can be replaced at once, but one should be cautious about that.)
Agree with the other Carbon comments. Carbon in average situations is only useful for about 3 days and then is ready to be trashed and replaced, which is why most aquarists only use it for the temporary special purposes it is good for: mostly removing medications after they have run their course. It is also sometimes used to help remove yellow wood tannins or the occasional organic odor of unknown origin. Overall, carbon is a good tool for the aquarist to have on the shelf. The scientific section once had a great thread explaining the whole carbon leaching thing and why is probably never an issue, as BBA mentions.
Also agree with FF and lockman that your reading of zero nitrate might be the result of an inaccurate test. Nitrate testing is notorious among organic chemists I believe for being a pain, especially if one tries to do it with inexpensive home equipment. It's not really anyone's fault, just the way the state of the art still is. It is very easy for one of the reagents to have the precipitate drop out and it can clump very hard in the bottom of the bottle. You have to actually whack the bottle hard against a shelf or other hard surface sometimes to have it come free and finally respond to tons of shaking, eventually resulting in a test that shows nitrate. The reagent bottles have sometimes sat in the shop or warehouse before you buy them. Somewhere on TFF we have threads on how to figure out the API expiry numbers. Another route can also be to try ordering in a Salifert test - these all come individually and are also of high quality.
Agree about plants changing test result expectations at times. This most often happens when the plants cover 70% or more of the substrate surface, so we're talking well planted tanks usually for this sort of thing. Ammonia is the thing seen to be absorbed the most with nitate significantly less. Nitrite(NO2) can also be absorbed by plants but that is rare and usually only in small amounts.
Once you get your Nitrate(NO3) testing worked out then (at least in the beginner section world where I usually hang out, lol) the question arises as to what "rules of thumb" there are. Well, one that we often mention to beginners is that we'd like to see maybe 15 to 20ppm -above- whatever your tap (or other source) water measures for Nitrate (or less if you're lucky.. these are meant more to be upper guidelines.) Anyway, let's say you fall somewhere in that range. The next important thing is not to get too interested in any one test result but to watch (in your aquarium notebook (you do keep one, right?) over time and get a feel for your Nitrate baseline. The baseline should be established during a period of months when you feel you are giving your tank good care.. weekly gravel-clean-water-changes etc. Once you've got this feel for your Nitrate you will be able to later -know- when it is creeping up on you and usually you will also know that your care has not been as good (by far the most common trouble, not having time to do maintenance) or perhaps you've added too much stock or an initial overstocking of young fish has grown older.
Note that how nitrate will bear on any individual species varies by huge amounts. Some fish (GBR's spring to mind) have reputations of being pretty sensitive to excess levels, whereas other types of fish might care much less (some river catfish are known to be quite happy in 400ppm nitrate or more.)
I see I've gone on, so apologies on that. Also there are of course many here who may remember bits better than me or are experts in some of the areas, hopefully they'll spot anywhere I got rusty!
~~waterdrop~~
should i tanke it out? it came with my tank and i assumed i should use it 
i will replace the carbon one peice at t time and i will try with my nitrate test again after bashing it even more, if then it doesn't work, i will look into buying a new test kit
