2.0 ph swing in water!!!

[87ford]

So I did a water change on my freshwater tank and noticed my ph was down way to far so I tested my tap water cold it was 6.8 and then for fun measured my hot water and off the scale it is 9.0++ my ro water measures 6.4 the kh gh for all is undetectable all 1 my question is what is causing such a high jump in ph in water I tested for copper got none it concerns me cause a mix will bring it at the right level around 7.4 but I added cold water to the tank to match the temp cause put it in too hot and today noticed ph was at 6.2 and freaked lol idk if posting in right area but figured this is chemistry on a side note is 6.4 ok for ro water? My saltwater tanks are stable at 8.2 but that's because of the live sand and salt mix I'm sure

Thanks
Tom

 

[disc1]

6.4 is about right for RO water. IF this was a saltwater tank I'd tell you not to worry, the salt mix will take care of it entirely. But you said this is a freshwater tank so pH is a much bigger concern to you.

What kind of fish are in this freshwater tank? I think the answer will depend a lot on that.

I think I would expect the pH of your hot water to be lower. But it's late and I've a little more than a little scotch in me tonight and I might be overlooking something. For perfectly neutral water, pH goes down as temperature goes up. At 100C the neutral point is close to 6. Maybe it has something to do with the scale and scum that always builds up in hot water heaters.

 

[Randy Holmes-Farley]

Many people have normal tap water pH quite high (can be >9, mine is). It is raised by water authorities to reduce lead and copper dissolution into the drinking water from pipes.

I'm not sure why you got higher pH in the hot than in the cold, but the hot water heater might drive off CO2, and, as mentioned, temperature impacts pH directly if you measured while it was still hot.

 

[dkeller_nc]

Your observations are fairly normal. Particularly in older homes, the cold water side of the plumbing can develop a fairly heavy biofilm, particularly if the pipes aren't flushed regularly by heavy use and thus frequently exposed to chlorine/chloramine. The bacteria in the biofilms generate organic acids and CO2 as waste products, which lowers the pH.

As Randy noted, all municipal systems target pH >8.3 for the reasons stated. On the hot side of the plumbing, the regular flushing of >110 deg F water tends to sterilize the piping and prevent the growth of biofilms (and the higher temps drive off dissolved gases, including CO2).

All this said, you ought not to be using straight RODI in your freshwater tank. It has almost no buffering capacity, and the life in the tank can easily drive the pH to extremely low levels. There are many buffering formulations available that's designed to counter-act this effect, but still leave the water low in hardness for softwater tropical fish, such as discus & tetras.

 

[TFP Marine Biologist]

pH of 6.8 is on the low side for tap water, I would suspect that as has already been mentioned, CO2 in the water is suppressing the pH. Leave an uncovered container of water on the counter overnight an see what the pH is the next day after dissolved gasses have equalized with the air. I would guess that the pH will then read much higher.

pH of 6.4 for your RO water is not out of the ordinary, you should add some buffering ability back to this water for freshwater aquariums, or you are asking for stability problems. There are many products available depending upon what kind of freshwater fish you are keeping.

 

[87ford]

I use straight tap for freshwater I have a semi agressive tropical tank needing about 7.0 ph I brought it up doing some baking soda but I was just shocked to see the hot and cold that diffrent I will latter today test that leaving it out for gas exchange I live in a apartment and it wouldn't surprise me what is in that water heater those lines are prob well over 40 years old if I had to take a guess surprisingly there is no nitrite nitrate and ammonia in my tap I tested it multiple times with different test kits so if that's the case it's pretty descent but the phosphates are at .25 but on a side note does ro system remove phosphates ? Or is that the rodi the di that removes it cause I have .25 in my ro water ? I'm using the api test kit for the phosphates this was all just a little game of knowing lol I'm very observant and prob test way to much went through 2 test kits in a month lol my only concern was what could be causing that much of a ph swing from hot and cold your saying c02 but that would bring ph down right so the cold water could have c02 in it correct or opposite

Thanks
Tom

 

[TFP Marine Biologist]

Yes, CO2 in the cold water is possibly suppressing the pH out of the tap.

 

[87ford]

Ok I will let the cold water sit tonight and then test it

Thank you
Tom