I have 2 C-530 canisters running on my overstocked 100G

Only water parameters will tell for sure. Do whatever you have to do to get that 0ppm NO2, 0ppm NH3, <40ppm NO3 reading. If you're severely overstocked, no amount of bio/mechanical filtration is going to help you deal with the nitrates.

rmkblades said:

My nitrates before the addition were always around 60 ppm, even after an immediate water changeS. I just added an extra c_530 MARINELAND so i will post future readings.

SO are you saying having 1 or 2 C-530 canisters wont make a difference?

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With respect to nitrates? No, it will make no difference. Biological filtration is there to convert ammonia to nitrite to nitrate. Mechanical filtration is mostly just there to make your aquarium look better (and of course to avoid clogging up your biological media with detritus). The only things that directly remove nitrates are anaerobic bacteria (which you generally don't want in your aquarium to begin with), and water changes. If you aren't manually removing it at least as fast as it is being produced, it will just build up.

rmkblades said:

that's why i do 80% water changes every week. I was just wondering now that i added another canister if my water parameters would improve.

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Extra filtration will help you keep your NO2 and NH3 in check, so in that respect, yes, the added canister is going to help you out, but you still have nitrates to worry about. A severely overstocked aquarium with adequate filtration is going to be a nitrate factory, so you just have to make sure that your water changes are keeping up with the nitrate production. Like I said, that will be determined by your water tests.