It depends on how high they are. How high is "high?" You may need to do larger water changes than that depending on how high they are. For example of nitrite is 1, you want to get it under .25, so you'd need to do around three 50% water changes to get them down. Ideally these would be done back-to-back. Doing them over the course of a few days just allows your fish to swim in toxins that could injure them.