How often should water be changed when cylcling a new tank

You should monitor the water daily for ammonia and nitrites with a test kit. Liquid kits like from from Aquarium Pharmaceuticals are best. The instructions should tell you what levels to keep these compounds below. If they exceed the limits, you need to perform water changes until they're back in a safe range.
 
A tank that small with that many fish will build up toxic levels fast. But as was said test and do water changes if you register any ammonia or nitrites. And a tank that small is easy to do 50% water changes in.

The fresher the water the better.
 
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