Just look at nature....fresh water is CONSTANTLY renewed by rains and snow melt. Consider the creeks, streams, and rivers that flow constantly towards the sea. For the aquarium, the fishkeeper makes the rain with routine periodic partial water changes....and there is rarely too much thing as too much fresh water!
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Too many hobbyists think their filters clean the water, but
The Dirty Truth About Filters is that they merely trap detritus where it decomposes and pollutes the water. Oh. filters are good for clarity, but not purification. And even with a good substrate loaded with good biology and fast growing plants, NOTHING beats replacing polluted water with fresh water.
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Now if we were able to mimic nature we would have fresh water trickling into the tank and overflowing so the water is always 'fresh' and less polluted. But this requires plumbing and drains and just not convenient for the living room show tank. The next best thing is the periodic partial water change of sufficient volume and frequency so as to keep the fresh water fresh...and often more water and/or more frequent is much better than too little, too late.
[end rant]
Regular Partial Water Changes
Partial Water Changes
Partial Water Changes - Fast & Easy