The fact that there is still room in your tank, or that you have had more fish in there than you have now, does not mean you are not overstocked. Technically, you *could* raise a litter of kittens in a closet, but I would not recommend it. Second, here's the thing about water testing: if you have algae, then you likely have nitrates. Your tests are reading zero because the algae is using the nitrates up. Doesn't mean they aren't there.