Tank Ph Dropped Really Low

[MoonstruckMuse]

My 125g tank which has been stable at about a pH of 6 for a few months now, has had a drastic-ish pH drop to a pH of 4.5. We haven't changed anything, but our 18" shovelnose cat mysteriously died around then. We have pretty heavy filtration on the tank and it is fully cycled, are very lightly planted, a light bioload, and what seems to be good aeration.

Also, we've done a 20% water change with almost no change in pH. Our tap comes out at 6.6, and all the rest of our tanks have remained steady at around 6-7. Oh, and I should mention that I wanted more SA cichlids in here, and intended to keep the water softish for them - not sure how the average pH raising things work with that.

What could be the cause of the pH drop? Is it what potentially killed the catfish, and/or did his death add to the dropping?
Can the arowana and severums tolerate this pH? Is it dangerous to them? How fast should I be changing their water to try and return the pH back, without causing a pH shock?

 

[rdd1952]

With a tap pH of 6.6, it is likely that your KH is nil meaning you have no buffering capacity. It's hard to say what could have caused the pH to have dropped that much. The fish death could have been the cause if you didn't noice him for a while and his body started to break down and produce ammonia thus requiring more bacteria, using more oxygen and lowering the O2 level. Are you sure it wasn't already that low ? You said that it had been around 6.0 for a few months but that is the bottom reading on most test kits I've seen so how would you know it wasn't lower? I assume you have a test kit that can measure lower than that.

The arowana and severums both prefer soft water so they shouldn't have too much trouble with it but I have never kept them so I don't know how hardy they are and how they tolerate non-optimal water.

 

[Davidtq]

What sort of substrate do you have in the tanks?

Quite often with acidic water the substrate \ rocks act as a buffering material until the calcium from them is depleted and then the ph crashes. It can take a good few months for that to happen.

 

[fishwatcher]

I've also read the high nitrAtes will cause a ph crash. Not sure how reliable that info is, but you may want to keep a close eye on the nitrates for a while (besides your normal testing/water changes).

 

[Aphotic Phoenix]

What is your normal water change schedule like?
Do you check your filter pads for debris build-up?

 

[ArauraDiscus]

With a tap pH of 6.6, it is likely that your KH is nil meaning you have no buffering capacity.
=================================

Well I have a h of 6.2 and a Kh of 300 ppms. A low Ph doesn't automatically mean your alkalinity is low. ALthough many people seem to think high Ph and high alkalinity, and low Ph and low alkalinity go hand in hand, there is actually know relationship between the two. A low Ph is likely due to some reduced buffering capacity, I agree with that. Also it could be from various things introduced to the tank.

And high Nitrates don't really cause your Ph to fluctuate or crash for that matter. Firstly, Nitrate is an anion and has a negative charge. Negative charged molecules would raise your Ph not lower it in most cases. Also I had a tank with Nitrate levels of around 300 for over 3 years, and the Ph was always around 7.2.

 

[rdd1952]

Low pH and low KH don't always go together but they seem to most of the time, especially when municipal water supplies are concerned. Most water companies try to keep the alkalinity as low as possible to make the water taste better. I agree though that it is very possible to have low pH and high KH and vis versa.

 

[MoonstruckMuse]

Sorry about the late reply, I had been out of the country for a little bit, heh. ^^"

I do in fact test it lower then 6.0pH because well, my dad's a chemist and he gets as many pH tester things as anyone could use in a lifetime. And I always got annoyed when it went off the scale and therefore, I checked it with the other tests even months ago.
I've got pea gravel on the bottom - I don't think it should be a buffer, but I'm not sure on that. I thought the usual gravel didn't really increase hardness of the water? I've also got two pieces of bogwood in there, which were pre-soaked to get rid of the tannins cause my father thought it looked "nasty" if it turned the water darker. I think those lower pH a little, but surely not drop it a whole point and a half, right?
My catfish was still alive, though he... appeared to have lesions all over his body and be bleeding slightly. Still not sure what killed him, but I don't think he was decomposing just yet - wasn't floating at that point, no fungus on the wounds, etc.
I usually change roughly 20% every few weeks, haha. ^^" I'm semi-lazy about it. However, the tank is pretty overfiltered... I use the largest canister filter fluval makes, and a sump that holds about 13g of water.

Can I just call the town and get water hardness numbers from there? And will my fish mind the harder water, should I choose to buffer it?