A tip to lower levels is to do larger pwc back to back or separated by a few hours, doing a 15% water change may help for smaller levels of toxins but the higher levels need large changes to properly dilute them. But you can always just change what you want, test the parameters again then adjust your pwc amount as you go, this is best I think.
Ps, don't ever listen to a pet store worker, really ever. A petco fish dept worker once told me he bred angelfish and several other types of fish, then proceeded to tell me all the fish I bough would probably die because fish are extremely hard to keep unless adult sized when purchased.
