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What percentage of water should be changed when cleaning tank and making regular water changes?

10% weekly is best.

Depending on what type of filter you're running on the tank you should plan a way that the filtration sponges/balls/rings/floss whichever is being rinsed out in the used tank water but not all at once.

For instance on one of my tanks I'm running a HOB at one end and a canister at the other. Instead of cleaning both filters at once, I'll alternate sponge from HOB then canister. This prevents too much of the bacteria being removed from the tank at one time.

When doing a gravel vac only do one third of the tank at a time. Again, for the same reason that you don't want to be removing too much of the beneficial bacteria at a time.

HTH

^_^

I do 50% weekly. Higher than that, water parameters are pretty unstable, much lower and I don't feel like I'm having much effect. I also wouldn't want to change that much water less frequently, because the fish may not be able to cope with such sudden changes.

I have heard of breeders and such doing 50% daily or twice daily water changes. This is probably not necessary unless you have a situation where the tank is very overstocked (I wouldn't start out with this much with a tank that has been sitting without a water change in a long time. It's something you'll want to work up to over a few weeks until you have more "new" water), or if you are growing out sensitive fry.

With larger aquaria, water quality is more stable and you can often get away with smaller changes. I do 50% for my tanks up to 20 gallons.

If you do some math you can see that larger water changes have a greater effect on the freshness of the water over time. For example, my calculations show that over 5 weeks of 50% water changes, I'll have 3.125% of the tank filled with the water I started with. However, after 5 weeks of 10% water changes, I will still have 59.049% of the original tank water!

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Hi,
Sup is correct with his math and the efectivenes of water changes. The idea with water changes is to dulute the "soup" that the fish swim in and maintain it in the cleanliness of their natural environment.

One reading you might look at is the Nitrate reading, generally in planted tanks, you would want to keep the nitrate reading around a 10, and less than that in a non planted tank. To maintain that, you would need to use a gravel vacuum on the gravel in the tank. In a planted tank, divide it into four sections, and with each weekly water change, vacuum a different section, that way, when a month is complete, your tank will have been cleaned as well the water.

Frank

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