If you have an ammonia or nitrite reading above 0ppm, do a 75% water change every day until the level is 0ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
The water doesn't hold much, if any beneficial bacteria. it is however, home to lots of harmful disease organisms. To reduce the disease organisms you should change at least 50% of the water when you do water changes and 75% is better.
You do water changes for 2 main reasons.
1) to reduce nutrients like ammonia, nitrite & nitrate.
2) to dilute disease organisms in the water.
Fish live in a soup of microscopic organisms including bacteria, fungus, viruses, protozoans, worms, flukes and various other things that make your skin crawl. Doing a big water change and gravel cleaning the substrate on a regular basis will dilute these organisms and reduce their numbers in the water, thus making it a safer and healthier environment for the fish.
If you do a 25% water change you leave behind 75% of the bad stuff in the water.
If you do a 50% water change you leave behind 50% of the bad stuff in the water.
If you do a 75% water change leave behind 25% of the bad stuff in the water.