Good question that will be different for everyone depending on stocking levels, fish size, amount of food fed, water parameters, etc.
Some members base the percentage of water changed on their aquarium water test results, an example would be seeing a nitrate level increase higher than the when the tank was first established.
Some members choose to perform weekly 50% water changes regardless of nitrate testing.
Every tank set up is different and it is often cited to keep the nitrate reading between 10PPM and 20PPM though that is not always possible for those that start with a nitrate reading from their tap (source) water.
However, just basing water changes and/or percentage changed is not just dependent on the nitrate reading. Most fish do appreciate the influx of new (fresh) water as there are some compounds or byproducts that we cannot test for.
For Africans I change 70%-75%/change the polishing pads in the sumps, weekly. Both my tanks are moderate to heavily stocked right now. When my 450gal was lightly stocked (with twenty-one 2"-2.5" Africans) I changed only 40%-50% weekly. Keeping Discus I changed 80% daily, so it also depends on which fish you're keeping as well.
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