Sudden PH Drop

Hi

After doing my weekly routine water tests, I noticed that the pH has dropped in my 105 litre aquarium from 7.6ish to 6.8. I cannot work out why.

The only changes I have made to my aquarium since the last test was add a new UV filter (which also adds air bubbles to the water) and a net increase of 2 bronze corys (I purchased 3 to replace the 1 that died last week, I thought that they might be happier in a group of 6). Some bag water did get out when acclimatising the cories, but it was only around 20ml.

It is a fully cycled aquarium which seems stable. I test at least weekly, sometimes daily after adding new stock/making changes. There is no sand or driftwood in this aquarium. I have even checked the pH test is working correctly by testing tapwater and other aquariums and it seems correct for those.

Does anyone have any ideas what could cause the pH to drop like this?

A few questions:

How hard is your water? You should be able to find this somewhere on your water provier's website. You need a number and the unit of measurement, though there are 3 companies that don't give a number. The reason for asking is that soft water usually goes hand in hand with low KH; KH buffers the water against changes in pH but where KH is low it can get used up leaving nothing to buffer the water.


How well stocked is the tank, how often do you do water changes, and how much do you change?
The natural tendency of a fish tank is to become acidic. Usually KH removes this acidity but when it is low, it can get used up. When there are a lot of fish, more acid is made. When water changes are done infrequently, the acids build up and can use all the KH. When water changes are not large enough, the KH does not get replenished so gets used up.


Do you have any live plants - and what time of day did you test pH? Plants take up carbon dioxide when the lights are and make carbon dioxide while the lights are off. Carbon dioxide dissolves in water to make a week acid. When the lights are off, the carbon dioxide made by the fish and plants lowers the pH so the pH is lowest just before the lights come on. The highest pH is just as the lights turn off as the plants have taken up most of the carbon dioxide.

Welsh Water is one of the three which don't give a number - and mine is another.

40 ppm KH is low. That's 2.2 dH which is low enough to allow pH to fall. I first found the forum (in its old incarnation) when looking for an explanation of why my pH had dropped off the bottom of the scale and the adviser who was here at that time said it was because of my KH at 3 dH. He told me to add remineralisation salts but I just changed my maintenance regime. I was lazy about doing water changes, and once I started doing 50% weekly, I've not had a problem.


The light in itself should not affect pH. Lights can indirectly affect pH by affecting plant growth.

I think it is just one of those things, caused mainly by the low KH. With such a low GH you need to keep soft water fish, and soft water fish won't be harmed by low pH.

What is the pH of water that's been allowed to stand 24 hours?

If it is similar to the tank pH, you could always let a container of water stand overnight - though in my case that's not possible as much husband would object. Loudly. 

I have just tested the pH after the lights have been on few hours and it's 7 now, so perhaps the live plants are affecting it a bit!

I suspect something has changed within my tap water, as it is happening to my other aquariums as well now! I remember testing the water hardness of my tap water when I moved here a few months ago and it was on the borderline of hard and soft, now its really soft, and the pH was stable in all my aquariums up until a week or 2 ago.