Max pH Change at Water Change

[tlkng1]

I really should already know the answer to this question.

My tap water pH is 7.4. I have, however, been using RO/DI water to make the water changes and pH there measures 6.4. Preferring a nice level pH of 7.0, I usually do a half RO/DI, half spring water change (pH 7.0).

Usually after I wait a few hours and do another pH test, the pH will be at 7.0 but at testing prior to doing another change a week later, the pH will be back down to 6.0. I would imagine with the water I am using I shouldn't have a hardness issue yet I can't keep the pH up.

The fish seem to be fine at the 6.0 range..at this point I have five swordtails and two cories. The tank has a few natural plants and a small piece of driftwood that I boiled several times until no more of the tannins were coming out.

The main quesiton in all this is how much can the pH change during the water change to keep from stressing out the fish too much? My memory right now is saying no more than a .2 change but I can't remember for sure.

Thanks

[eco23]

Why are you using RO/DI water? 7.4 is a perfect range for virtually every fish other than very sensitive ones like Discus. All fish will prefer stable pH no matter the range over fluctuating levels. The RO also strips all of the buffers out of the water which will help maintain a constant pH level.

[Fishguy2727]

Even discus would be fine.

[MartinBlank]

In chasing a puzzle of my own I have been researching this a lot just recently, so I will take a stab. Someone of course correct me if I am wrong.

It depends on the kH of the water. Water with high kH resists changes in pH (either up or down). When kH is low, it doesn't take much to change the pH at all (and you can have larger swings).


RO/DI water will have a kH of 0. No idea what your spring water will have (did you mean tap water?), but if it also has a kH of 0, then you are going to have a lot of trouble maintaining a stable pH (just the fish / plant / food waste will push it downwards).

Now tap water often has a reasonably high kH. Though about mixing RO/DI with tap water 50/50? Or is that what you are doing?

[tlkng1]

I use the Kent RO Right which adds back the minerals that the process takes out. I can work the 50/50 RO with tap but right now with the pH so low, I think even a 50/50 at this point would raise it too far too fast.

From experience, my tap water, even treated, isn't the healthiest for fish.

Any idea what the safe change in pH might be?

[eco23]

Originally Posted by tlkng1

I use the Kent RO Right which adds back the minerals that the process takes out. I can work the 50/50 RO with tap but right now with the pH so low, I think even a 50/50 at this point would raise it too far too fast.

From experience, my tap water, even treated, isn't the healthiest for fish.

Any idea what the safe change in pH might be?

I don't know the exact safe range (I've heard .2 I believe?), but personally I'd do ~5-10% pwc's every few days until the value is brought back up to it's natural level. What do you think is unhealthy about your tap water? Are you using a quality conditioner like Seachem Prime?

[Fishguy2727]

Your tap water would have to be VERY bad to not be okay for fish, probably illegally poor.

What problems did you have that indicated the tap water was the problem?

[tlkng1]

Experience mainly. I would get explosions of algae at times even though nothing changed..lighting remained the same as did feeding..I feed light.

When I switched to RO water or spring water, I didn't have the algae problem any more, again, no changes in general system. No lighting difference (lights were on timers), no temp fluctuations beyond a degree or two, no issues with plants dying, no changes in feeding.

[Fishguy2727]

What was the water change schedule at that time?

[tlkng1]

On a 75g, 20% weekly.

[Fishguy2727]

That water change schedule is pretty light. Depending on the stocking that may have been part of the issue.

[tlkng1]

...maybe...its just that the only thing that changed when I swapped to RO water was no more algae blooms.

I did a triple pass on the water change yesterday (10G). Used part tap, part spring and part RO. pH didn't change at all so the fish are happy today. I'll slowly start changing over to all tap and see what happens. It would certainly be a lot easier.

[Fishguy2727]

What is the nitrate concentration of the tank and tap water? What about phosphate?

[tlkng1]

Nitrates are reading 0 for both...can the phosphate test used for a reef saltwater also test freshwater?

[Fishguy2727]

Depends on the test, what does the packaging say?

I doubt it is 0. What test are you using? Are you sure you did it correctly? Many are quite complicated and not following the instructions to the letter can result in false 0s.