Confusing ph

[Anthony1976]

Hello everyone just want to run this by you guys to see if you can work this out. My tapwater ph is very high around 8.0 ph. I have access to good clean rainwater which I tried mixing 50/50 with the tap water in a 1 litre jug. The rainwater is 6.2 but mixing it 50/50 only brought the ph down to 7.8. I tried 750ml rainwater in the next test with 250 ml tap water and the same result 7.8 ??? What’s going on here ???

 

[Essjay]

pH is not a linear scale, it is logarithmic. pH 8.0 is 10 times more basic than pH 7.0 and 100 times more basic than pH 6.0. And the minerals present in the tap water will influence the pH even at half and half mix. If the minerals are basic, the water will still have a basic pH.

What is the GH (hardness) of your tap water, and what species of fish? GH is more important than pH for fish.

 

[Anthony1976]

pH is not a linear scale, it is logarithmic. pH 8.0 is 10 times more basic than pH 7.0 and 100 times more basic than pH 6.0. And the minerals present in the tap water will influence the pH even at half and half mix. If the minerals are basic, the water will still have a basic pH.

What is the GH (hardness) of your tap water, and what species of fish? GH is more important than pH for fish.

Not sure of the gh I don’t have a test kit for that. I keep albino Cory’s , pristellas , glowlights, and Otocinclus, I haven’t lost a fish in over two years and the Cory’s spawn regularly. I kind of get what you mean about the minerals influencing the rainwater. So how do I go about lowering the ph with the rainwater ? Thanks very much for the reply and help

 

[Essjay]

Look on your water provider's website for hardness. If you have soft water there is not a problem with the pH. But if it's hard it is not good for the fish you have except maybe the pristellas.

But first see if you can find out the hardness, you need a number and the unit of measurement. Once we have that we can see whether anything needs to be done or not.

 

[Anthony1976]

So your basically saying it’s because the rainwater has no buffering capability?

 

[Anthony1976]

Look on your water provider's website for hardness. If you have soft water there is not a problem with the pH. But if it's hard it is not good for the fish you have except maybe the pristellas.

But first see if you can find out the hardness, you need a number and the unit of measurement. Once we have that we can see whether anything needs to be done or not.

Ok I will look into this thankyou

 

[Anthony1976]

Just looked online where I live and the gh is 178. I live in an area called the limestone coast as the name suggests lots of limestone

 

[Essjay]

I presume that's ppm or mg/l CaCO3 (same thing, different names). The other unit used in fish keeping is dH, and your 178 ppm converts to 10 dH. This is middling hardness - it's too soft for many hard water fish but too hard for fish that must have very soft water.

What species of fish do you have or intend?

 

[Anthony1976]

I presume that's ppm or mg/l CaCO3 (same thing, different names). The other unit used in fish keeping is dH, and your 178 ppm converts to 10 dH. This is middling hardness - it's too soft for many hard water fish but too hard for fish that must have very soft water.

What species of fish do you have or intend?

Have two tanks, lots of Cory’s bronze and albinos, couple of large saes , harlequin rasboras, rummy nose tetras, and the ones I mentioned before. Been up and running 2.5 years have lost only one fish in this time .

 

[Anthony1976]

Hardness
178 (mg/L) that’s all it says

 

[Essjay]

In that case it will be mg/l CaCo3, 178 dH would be solid rock. It's the UK that uses odd units most other countries use mg/l CaCO3.

All the fish are OK at 10 dH so you don't need to alter the water for them. The only way it could affect the fish is that rummy noses have redder noses at pH below 7.
These fish can also live in softer water so if you wanted to mix your tap with rainwater it would not harm them and as you say it would bring the pH down slightly. But you must have rainwater on hand at all times; you can never do a water change with all tap water once you've started mixing it with rainwater.

 

[Anthony1976]

In that case it will be mg/l CaCo3, 178 dH would be solid rock. It's the UK that uses odd units most other countries use mg/l CaCO3.

All the fish are OK at 10 dH so you don't need to alter the water for them. The only way it could affect the fish is that rummy noses have redder noses at pH below 7.
These fish can also live in softer water so if you wanted to mix your tap with rainwater it would not harm them and as you say it would bring the pH down slightly. But you must have rainwater on hand at all times; you can never do a water change with all tap water once you've started mixing it with rainwater.

Yeah I have a huge rainwater tank, still a bit confused as to why it isn’t dropping any lower than 7.8. So what you’re basically saying is the rainwater doesn’t have any buffering capability??? I though that removing the minerals with the rainwater mixed in would reduce the ph ? Sorry if I’m coming across as a bit stupid really appreciate the help

 

[Essjay]

Because you are mixing the rainwater with tap water you are diluting the minerals in tap water not removing them. The minerals in the tap water are still there, just not as many of them.
The minerals in tap water depends on what rock or soil the water has been in contact with since it fell as rain. If it dissolved minerals that cause high pH, they are still there when you mix it with rainwater which (in theory) is pure water with no minerals at all. So the mix still contains minerals which cause high pH.
If you were to use all rainwater, that would have no minerals at all in it so in theory the pH should be 7.0. I say 'in theory' because rain does dissolve things from the atmosphere as it falls which can alter pH slightly. This is the main reason why rain water shouldn't be used where there is air borne industrial pollution or agricultural pollution (eg crop spraying) and you don't want contaminants like that in a fish tank.
It is possible to use only some form of pure water (rain, RO etc) in a fish tank - we have members who do just that - but most people add minerals to the pure water which again, depending on the nature of the minerals, does change the pH.

 

[Anthony1976]

Because you are mixing the rainwater with tap water you are diluting the minerals in tap water not removing them. The minerals in the tap water are still there, just not as many of them.
The minerals in tap water depends on what rock or soil the water has been in contact with since it fell as rain. If it dissolved minerals that cause high pH, they are still there when you mix it with rainwater which (in theory) is pure water with no minerals at all. So the mix still contains minerals which cause high pH.
If you were to use all rainwater, that would have no minerals at all in it so in theory the pH should be 7.0. I say 'in theory' because rain does dissolve things from the atmosphere as it falls which can alter pH slightly. This is the main reason why rain water shouldn't be used where there is air borne industrial pollution or agricultural pollution (eg crop spraying) and you don't want contaminants like that in a fish tank.
It is possible to use only some form of pure water (rain, RO etc) in a fish tank - we have members who do just that - but most people add minerals to the pure water which again, depending on the nature of the minerals, does change the pH.

I get what you’re saying . My rainwater runs off my roof and I tested it at 6.2 ph which is why I was confused as to why my end ph wasn’t lower after mixing it 50/50. I do use a small amount of seachem equilibrium to replace minerals for my plants

 

[Naughts]

You were expecting 50:50 rain and tap to meet somewhere in the middle of the pH, like 7.1. I think this works for GH which is a linear scale (50:50 with pure water would be half the minerals which halves the GH) but not for pH which is logarithmic. I think part of what Essjay said in post #2 is that pH 8 is 10x more alkaline/basic than pH 7 so you would need ten times more rainwater than tap to move it down from 8 to 7. Sorry, I can't explain things as well as Essjay (especially not chemistry!).