Aquarium Help?!?

We recently got a 75 gallon tank that came with fish. We also have our own, but are trying to find new homes for many of them due to fear of over population. We have done a complete water change obviously when we first got the tank, and at least 3 partial water changes since. I test the water often, and have put in things to lower ph, ammonia, nitrates, nitrites, and produce bacteria for the filter. I'm pretty sure the filter is not completely cycled yet, as there is still a small odor from the tank. I've increased oxygen flow, reduced feeding and regulated temperature. My question is: why are my bottom feeders still having rapid gill movement, and some fish hug the top of the tank as if gasping for breath? I know these are signs ociated with nitrate poisoning, but I watch the levels and can only use so many chemicals before I wonder why they aren't working. Is it just that my filter has to finish cycling? Or is there something else I could do to aide to my fish?? I want them happy!