How much does a water change effect ph?

Question

I have a 30g and its at 6.5ph atm. If I change like say 20% of the water and the ph of the changed water is 7 how much will it effect the water in the tank

Answer 1

Let's put it this way, let's say the water has a pH of 6.5, but the tap is 7.0.
If you change 20% you have 20x7+80x6.5 then divide by 100.
140+520=660 divided by 100 is 6.6.
Using chemicals to alter the pH cause sever fluctuations which kills fish. Most fish can adapt to a range of pH, so long as it is stable.
The only fish you might need to mess with chemicals are African Cichlids. Say your tap water is 7.0 this is too low for most African Cichlids. You could use a little bit of crushed coral in your filter to raise pH and hardness, as you probably want 8.0 or greater. Discus would prefer to be in water with a pH of 6.0 to 6.5, but they very well could have been raised in water that has a pH of 7.0. You might want to filter with peat if your pH was 7.5 or higher in a Discus tank.
Most other fish will be fine, so long as it is stable.
You can start playing with buffers. Kent, SeaChem and Aquarium Pharmaceuticals make some good ones. Some people have luck with them. Some don't.

Answer 2

There's really no way of predicting that without knowing the carbonate hardness/alkalinity. The pH just tells the concentration of H+ ions in the water. The buffering capacity provided by carbonates causes the pH to resist change.

Let me use an example I used to do for Science Olympiad when I was teaching to explain this a little better. In the water quality event, students had to show they knew the relationship between pH and alkalinity/hardness/buffering capacity. I set up two 10 gallon tanks (lakes) and had the pH of one at 6.5 and the other at 7.0. They tested the pH of each "lake" then put 15 drops of diluted vinegar ("acid rain") into a beaker of water from each lake and tested for the new pH. The lake with the pH of 6.5 changed to 6.4, the lake that had a pH of 7.0 changed to 5.8. Which lake had the better buffering capacity? The lake with the pH of 6.5 because it showed less of a change when an acid was added, even though it had a lower pH to start.

Even though these parameters are related and they influence each other, they aren't the same thing. If you tank water has a high buffering capacity, it may change very little. If it doesn't, it could change a lot. That's why I don't recommend the Ph up and pH down - how well they work depends on your water's buffering capacity, and they can cause your water to fluctuate too much.

If that wasn't enough of a science/chemistry lecture for you, see this link: http://faq.thekrib.com/begin-chem.html

Answer 3

I wouldn't think a 20% water change with .5 difference in pH would affect the tank's pH too much. It's usually best to leave the pH in your tank the same level as your tap water, that way the pH always stays pretty much the same. I have a Discus tank. Discus are very particular to to pH levels. They actually like a lower pH level than my tap water, but since they are even more fussy about pH changes, it's better to just get them used to the default pH level in my tap water so I don't have to worry about lowering the pH level of the water every time I change water (which is very often). I have an 80 gallon Discus tank and do a 40-50% water change every week. I'd go broke in a hurry if I had to buy pH lowering formula for water changes. A pH level of 7 is fine for practically any kind of fish I can think of. My pH is a bit higher than that and even the pH-fussy Discus seem to do just fine with a pH level of 7+.In short, it's better for fish to get used to a higher pH level than recommended for that species than it is to subject them to pH changes. Hope that made sense and helps.

Answer 4

not that much -- but its part of why you only do partial changes. It would be worse in my opinion to start dumping chemicals to reduce the ph of the fresh water.