I am assuming the question is with reference to a freshwater aquarium (not marine). If fish are in the aquarium (or intended), you need to do a regular--which here means once a week, every week--partial water change. The volume of the water changed is where aquarists vary. While it is true to a limited degree that this depends upon the number of fish, tank size, live plants, fish size and type, and such, the scientific truth is that the more water changed the better the health of the fish. Minimal volumes, like 1 gallon a week will make absolutely no difference; 10 gallons in a 125g similar, even 25% of this volume (which would be roughly 30 gallons, is really not going to do anything. I'll come back to this.
Provided the parameters--and to be certain, these refer to four things only, GH (general hardness), KH (carbonate hardness or Alkalinity), pH and temperature--are reasonably similar between those of the tap (source/fresh) water and the tank water at the time of change, large volume changes of 50-60%, even 70% of the tank's water can and should be changed. The article pinned in the Tropical Discussion Form here
https://www.fishforums.net/threads/regular-partial-water-changes.471488/ explains why this is good maintenance. There is "stuff" in the water that cannot be removed by any other method.
The percentage of the volume of water in the tank determines how much of the crud or pollution gets removed; if you remove half the water, you remove only half of this crud/pollution and the other half remains, increasing obviously until the next change. Removing say 25% of the tank's volume is only removing 25% of the bad stuff. To be really effective you need to remove as much as possible. There is no way to measure any of this for most of us, it is just scientific fact--and a good habit to get into from the start!
