Crazy Changes In Tank Ph

[NewTankGuy]

I dont know how this is happening, so I was hopping one of you could help.  
 
My tank normally has a stable pH of right about 7.0, however, about a week ago it has dropped, and is now holding steady at 6.0.  Since the drop, it is always at a pH of 6.0, so there is no up and down going on, even with 2-3 w/c's a week, its just fallen and cant get up...
 
Now here is the interesting thing. When you test the pH right out of the tap, it maxes out the pH scale on the API master test kit at 8.8 and looks like a much darker purple than that even.  Also, I live in Toledo, OH and  this change in pH directly coincided with our recent water crises where we could not bath, drink, or cook with our tap water because of a massive algae bloom in Lake Erie.  I waited till about 3 days after they gave us the "ALL CLEAR" to go back to normal water usage before I changed the water in my tank, but thats when these issues all started.
 
So my question is, how do I have a pH of 8.8 or higher coming out of the tap, but even after a 20-50% w/c the pH is 6.0 (or lower) in the tank?  
 
The only thing I use during w/c's is my Prime water conditioner, so i dont use any pH chemicals at all as i heard they only lead to problems latter. 
 
In my 75 gallon tank, besides the fish, I have 7 good size live plants (with 4 more being added tomorrow), and a large piece of drift wood. Which all have been in there since I started the tank.
 

 

[Byron]

One very important piece of data is missing, and that is the carbonate hardness, termed KH or Alkalinity, of your tap water; tank water will be much the same, unless you are somehow altering the chemistry (deliberately or not) which doesn't seem to be the case.  You should be able to ascertain the KH from the water supply people, though with all the issues it may not be accurate...or you can test it if you have the API GH/KH test or comparable.
 
I am suspecting your tap KH is very low.  This means that once the aquarium is established with fish, the organics will begin to accumulate and during the breakdown by bacteria CO2 is produced, quite a lot actually in a healthy system.  This creates carbonic acid and the pH lowers since there is no buffering capacity (the KH) to prevent this.  This can become quite stable in itself, and water changes provided they are not too major have little impact.  I have this very situation, which I am grateful for as I keep all soft water fish.
 
I may have more when I know the KH, but for the present I would presume this.
 
Byron.

 

[fluttermoth]

Also, are you leaving the tap water to stand for 24 hours before you test it? Many water companies temporarily adjust the pH (usually to prevent corrosion), so you need to let that wear off before you get a 'true' pH.

 

[Byron]

Excellent point.  And related to this, the amount of CO2 dissolved in the tap water can affect pH considerably.  This too will dissipate in 24 hours (if there is any), or you can very briskly shake the water to out-gas the CO2.  B.

 

[TwoTankAmin]

During water emergencies it is common for water companies to artificially change the pH in the system because it facilitates the treatments they need to use to correct the problem. So they may alter the pH in a fashion it ends up safe to drink but my be a radical departure from the params the fish are used to.
 
In your case if the problem is related to co2 it would be a lack of it out of the tap and the reintroduction of into the water once it is in the tank and being agitated. However, my guess is they used something besides controlling co2 to handle the issue.
 
If you can contact the water company and ask about this you will likely get the best answer, Short of that you might want to use a buffer temporarily until things return to normal. The biggest risk is that your water is lower than 6.0/ Most hobby kits do not read below that so a 6.0 reading could be just that or it could be your water is much lower. I run an acid water tank and have had it as low as 4.2 pH. To test below 6.0 I have to use digital methods.
 
Finally, if all of this has caused anything relating to the cycle to go off such that you have ammonia, the pH level where it now is protects the fish from the effects since NH3 concentrations drop substantially at your pH levels. If the pH then is raised and there is ammonia, the raising will turn it much more toxic fairly quickly.

 

[NewTankGuy]

Thanks for all the tips, and help! This helps to make a lot more sense of everything! I appreciate it!