pH went up after a 70% water change with RO

[AlexT]

If your pH goes up from 7.2 to 7.6 after a water change, then basically, the water that went out was more acidic than the water that was going into the tank? It's lucky I do my water changes quite slowly anyway, as it was at least a 0.4 swing on the pH after the water change. Would a 0.4 change all at once be a problem for some or most fish?

@Colin_T you already did mention something to me about my plan to carry on with 80% RO and 20% Tap.

History of the numbers
Tap Water - pH 7.6 - KH 10 - GH 16
80% Ro & 20% Tap - pH 7.2 - KH 5 - GH 6

Today, the 70% water change was pure RO, which probably means my overall water in the tank is now 90% RO and 10% Tap (at a guess).

These are todays tank numbers about 6 hours after the 70% water change
pH 7.6
KH 2
GH 3

I was surprised that if all the water going in today has a pH of 7.0 (and it did, I tested the RO), then how comes my overall tank pH has gone from 7.2 to 7.6 ? Is it because my KH is now 2 and this will make the pH erratic? I also have a fair bit of leaf litter and wood in my set up, but not Almond, it's beech I think. The 70% water coming out was quite coloured with tannin.

Could the organic build up have made my tank water more acidic, and then when the 70% fresh RO went in, no wonder the pH went up?

What do you think of my current parameters and RO mix (either 80% or 90%) in terms of my fish?
pH 7.2-7.6 KH 2-5 GH 3-6
- Bronze Corydoras
- White Fin Bentosi Tetra
- Bristlenose

If I run my tank at 90% RO and a KH at 2, is this going to be a problem? Should I increase the hard tap water percentage to 20% again or more, to get the KH to 4 or 5 ???

 

[jaylach]

I don't know if this is true with RO water but with tap water you can't test PH straight out of the faucet. You have to let the water sit for a day as the PH will likely change.

 

[Byron]

RO has (or should have) zero GH and KH. The pH at 7, fine. There is no KH, so the pH is free to do whatever the water in the tank dictates. The pH will rise because the buffering in the tank water affects it.

 

[AlexT]

I did a 10% water change with dechlorinated tap water to be on the safe side. KH has gone up from 2 to 4, and GH from 3 to 5

 

[AlexT]

I don't know if this is true with RO water but with tap water you can't test PH straight out of the faucet. You have to let the water sit for a day as the PH will likely change.

I never knew that actually

 

[Byron]

I never knew that actually

Briefly to explain the reason...tap water can have dissolved CO2 in it, quite a lot. If you see lots of tiny bubbles after a water change, that is CO2 outgassing from the fresh water. I have seen this widely vary, depending upon the season, weather, etc, which all affect the dissolved CO2. In the pipes travelling to your house this dissolved CO2 stays in the water, it has no where else to go. Once you run it into the aquarium, the CO2 begins to out-gas. As it does, the pH rises back to where it is for the source water. Testing the tap water with the CO2 in it will give a lower pH reading. Letting the water sit 24 hours, or alternatively you can very briskly agitate it, dissipates the CO2 out of the water, so the pH rises to its normal level for the water.