PH swings during water changes?

I've never seen any associated stress, only good health from large water changes, and the pH changes are typically 1 full unit of pH.

This has been occuring universally for the last 15 years in all my tanks.
And most others..........

CO2 is causing the decline in pH, Not less TDS/Salts, buffers etc.

CO2 is not a salt last I checked nor is it a buffer.

Blood systems are trying to unload CO2 efficiently in fish, so removing say 40ppm of CO2 in the water and adding only 5ppm is going to help, not harm fish in this respect.

Since CO2 is not a salt, this has little effect on osmoregulation.
pH differences does effect cell transpport, but that is an internal process, and shielded from the outside. Otherwise fish would be far far more sensitive to slight changes in pH and would die very easily.

Bottom line is do not worry about it.

There's a great deal of practical observation and evidence that suggest in a tank where CO2 is used, it does not matter directly.

Regards,
Tom Barr

My local water company adds sodium hydroxide to keep the pH up. I called and asked.
GH and KH hover around 4-5 degrees through the year, but the pH can be anywhere from 7.6 to 9.

Fish are more concerned with the mineral and salt levels in the water. If the new water has the same GH and KH and TDS as the old water it is not a problem that the pH changes.
I have had problems moving fish from one tank to another when the mineral levels are different. This relates to the osmoregulation that fish must do to stay alive. The contact through the gills of tank water and fish fluids allows the exchange of more things than just oxygen and carbon dioxide.

When there are more minerals in the water than in the fishes' tissues the fish loses some of its internal water to the tank. This raises the concentration of all the things in the fishes' cells and can kill the fish. The fishes' body compensates, and the levels maintain their optimum levels. This is the situation for fish in marine conditions. They have ways of keeping the water they need and getting rid of excess salts and minerals.

When there are less minerals in the water than in the fishes' tissues the fish gains water from the tank. This drops the concentration of all the things in the fishes' cells and can kill the fish. The fishes' body compensates, and the levels maintain their optimum level. This is the situation for fresh water fish. They hold onto most of the minerals, and get rid of excess fluid. Fresh water varies, though, so simply saying it is fresh water is not enough. Fish moved from hard water to soft, or from soft water to hard can have problems. Many years ago fish keepers had no way to test mineral levels, just pH. Often as mineral levels in the water change the pH also changes, so these fish keepers thought they had a pH problem. It is not a pH problem.

Problems happen when the mineral and salt level in the water changes too fast. If the levels change slowly the fish can adapt. Slow changes over several weeks (within the tolerance range of the species) are not a problem.
Fast changes (such as buying a new fish, moving fish from one tank to another, or a large water change) create too large a change in the water chemistry for the fish to be able to adapt.
Check the GH, KH, and TDS of the water the fish are adapted to, and test the new water you want the fish to be in. If these numbers are less than 10% different the fish will probably be fine. If the waters are too different you have several options:

1) Alter the water.
This is what bdement is doing adding things to the very tap soft water when the tank water has fertilizers and other things in it. When the levels are similar, the fish are fine.

2) Put the fish in a quarantine tank.
Make the Q-tank water match where the fish came from, and slowly change the water (take a month to do this) Then put the fish in the main tank. This is what I do when I buy new fish.

I too am concerned with pH swings or perhaps more accurately in my case, pH dropping! I'm using a (new) digital pH tester, re-calibrated this past weekend to 7.0. Water from our tap is now 9.2, a few months ago it was 8.2. I "age" the water (overnight) before doing a water change in two 35-gal plastic trash cans, that of course, have only been used for this purpose. I'm only adding 3 tsp of Amquel per trash can, adding a larger airstone to each and a separate heater set to 84 degrees.

When I test the water the following day, it is typically @ 7.2 - 7.4. (also tested with a liquid test kit) Quite a drop with just airation! A non-airated glass of water sitting on the counter may drop to 8.9 after the same period of time.

My 155 gallon planted Discus tank has 3" of Eco-Complete substrate (started in August 2010), 3 medium sized pieces of driftwood, a Rena XP3 and an XP4 canister filters just cleaned both within the past week, 3 days apart. The XP3 contains only filter pads and some bio beads/stars. The XP4 contains all of that, plus some (fresh) charcoal to help remove the tannins from the water. NO CO2. Liquid ferts twice a week, started this regime about 2 weeks ago. I tested the pH on Thursday of last week in the tank and it was like 5.6, I don't recall the exact number but in the upper 5's. I was a bit concerned to say the least! I whipped up two cans of water (~60 gallons total) Friday night and did a water change Saturday morning. I don't remember the exact numbers but it prompted me to set up another 60 gallons of water and did another change later that day. I tested the pH in the tank on Sunday and it's 5.2!!! I'm at a loss as to why the pH is dropping like this! The fish and plants all look OK, but still it concerns me that the water is becoming too acidic.

Thoughts/suggestions? Should I add something to the canister filters to help bring the pH back into the mid 6's? Water changes don't seem to be helping much. Thanks for reading my long description, hopefully I have provided enough detail.

PS: after reading the other replies, I need to test for GH and KH. I don't believe I have a way to test TDS.

Hoppy said:

If you believe that the tannins from your wood in the tank are causing the pH drop, you can add baking soda to the water to raise the KH. That will buffer the water better. If you already have water with a KH above 2 degrees or so, the only effect of the baking soda will be to raise the pH, not add more buffering. Is your tank water yellowish in color, from tannins? If not, I don't see how you could have enough tannins to give that effect, unless the tank water KH is below 1.0 degrees.

Click to expand...

I don't think the driftwood is causing it. I soaked all the pieces for several weeks before putting them in the tank, sometimes using hot water and changing the water daily. They've been in the tank now for 4 months. The water just had a slight yellow tinge. Now it is clear and I'm only running charcoal in one canister filter.

I just tested the GH and KH and both give me approximate readings of 5 degrees or 89.5 ppm. Looking back over saved data, these numbers are consistent with tank startup back in August of last year.

Just to throw in a curve ball...I have a quarantine tank (20 gallons) with a H.O.B filter that contains the Rena black bio stars and a filter pad. Nothing else in the filter. In the tank: an air stone, no substrate, no driftwood, nada. I drained it completely this past weekend (Saturday) and filled it up with 20 gallons of "aged" water. pH: 7.2, temp 82 degrees. Nothing added other than some Amquel. Sunday we picked up some new fish from several different stores, soaked the bags, acclimated the fish in the bags with water from the tank, never putting bag water in the tank, always dumping it out. Checked the pH when I came home tonight: 4.3!!! I immediately filled one of my water changing trash cans from the tap, added Amquel an airstone and a heater. pH right after filling: 8.4 @ 78 degrees. Let it sit in the trash can for an hour and did a 5 gallon water change. Checked the pH in the QT about 30 minutes later and it had climbed up to 6.2. I pulled out all the bio stars and left just the filter pad. The bio stars had been in use for just over a month. Waited about an hour and changed out 5 more gallons. Just checked the pH and it is: 6.8 @ 78 degrees. I sure hope the change up in pH won't send them into shock. I'll check the pH again in the morning when I get up which is @ 5:30am.

The Discus tank is at a pH of 5.4 @ 82 degrees as of tonight, still way lower then I would like. Both canister filters have bio beads and the bio stars. I'm real curious to see what happens to the pH on the QT.

UPDATE: back on Wednesday of last week, I pulled all the fish out of quarantine and slowly acclimated them to their new homes. Everyone as of today is doing fine. I completely drained the QT and at the moment it remains dry. I did two water changes (total of ~ 120 gallons) on my 155 over the weekend using "aged" water with a pH of 7.2. On Sunday morning, the pH was only 5.8. I went out to a LFS and bought 4 lbs of medium size crushed coral, not the sand. I shut down my XP4, pulled the charcoal out of one of the baskets and added aprox. 2 lbs of the crushed coral (rinsed) between two filter pads. This morning before leaving for work, I checked the pH and it's now up to 6.7. So over the course of about 14 hours, the crushed coral pulled up the pH almost one full point. I'll check it again tonight to see if it's climbed any higher. If it has, I'll probably pull out about a pound and see what happens. I'll do daily pH checks probably for a week to ensure it's not bouncing. I'm cautiously optimistic that I may have it figured out AND I'm not using chemicals to alter the pH. Crushed coral is cheap and easy to replace. Honestly, with my tap water pH coming out in the mid 8's or higher, I NEVER thought I would have a low pH problem. Live and learn.