Sudden rise in pH

[MarkT]

Hi Guys,
I have had a established 27 gallon shrimp tank now for 9 years.
I have kept the pH around 6.8 and it's been filtered with 2 large external cannister filters, all water parameters have always been fine.
All of a sudden the pH rises to 7.5 within hours of a water change.
Tap water is 6.7/8 if I do a 50% water change the pH goes to over 7.2 within minutes.
I have not changed a thing, no new introductions, no change in food, nothing.
I have Co2 injection controlled by a pH controller that has been fine.
Some mornings the pH has been bought down to 6.8 by the Co2 by lunchtime its back to 7.5.
Anybody have any ideas?

 

[LightingBamboozled]

What are the settings on your CO2 and pH controller. That would be my first suspicion because if it's meant to be keeping pH stable by adding CO2 then it's probably not adding as much or the plants are consuming more.

Do you add cuttlebone or any other source of calcium carbonate for the shrimp to maintain shell structure? Is it posible excess has been added recently?

 

[_Maq_]

No provider of tap water guarantees that the water will permanently keep the same parameters. They only observe health requirements.
There's also possible that oxygen demand in your tank increased to higher level than before. In such a situation, various nutrients (N, S, Fe, a.o.) turn into their reduced form and pH increases.

But in your case, probably some trouble with your CO2 stuff is the culprit.

 

[MarkT]

What are the settings on your CO2 and pH controller. That would be my first suspicion because if it's meant to be keeping pH stable by adding CO2 then it's probably not adding as much or the plants are consuming more.

Do you add cuttlebone or any other source of calcium carbonate for the shrimp to maintain shell structure? Is it posible excess has been added recently?

I used to add BiCarb when the PH was low, but stopped ages ago.
Never added any Cuttlebone or Calcium Carbonate...should I?

 

[Mat H]

I'd probably double check gassed off (left in glasses for a day) samples of both tap and tank water just to be sure.

Above said I think it may be the CO2, and the pH controller in particular, especially seeing as the pH appears to be dropped in the morning but increased by the afternoon - it maybe worth double checking the pH and pH drop yourself with some freshly calibrated equipment if you have any.