This is new. I have a well-established 75 gallon aquarium with six medium-sized blood parrot fish that I've had for years. Suddenly the pH of the water is consistently below what my testing system can even measure (< 6 pH). I've had to step up the water changes doing 30-50% 1-2 times/week because the fish are obviously stressed within days after the water change (clamped fins, sitting on the bottom, etc).

I've tested the tap water, even after it's been treated with Prime water conditioner; all normal.

It's a 75 gallon aquarium with a 350 gal/hr canister filter layered with 20-30 ppi filtration foam pads, a microfiltration pad, bio balls, and Seachem Purigen. I use no other chemicals. The tank has minimal ornamentation, no plants, and no gravel. Nothing else has changed recently.

Any ideas (besides doing way-too-frequent water changes)?

  • So the pH was testing normal before that? Commented May 6, 2015 at 16:54
  • @starsplusplus Yes, I've had no problems in previous years. The nitrogen cycle and bio activity has kept this system pretty stable for years. This need for constant maintenance is a new thing. Commented May 6, 2015 at 16:59
  • You need gravel , etc ( preferably limestone type) and plants to balance the acidic pH from carbon dioxide produced by the fish. I have accidentally killed fish by putting them into a bare tank as you describe. Commented Oct 15, 2018 at 1:00

2 Answers 2


The first thing I would check is the pH of the water you're using to fill the tank with.

You've said that you tested it already and it was normal, but it could actually be that the water is normal as it's coming out of the faucet and then later reverting to the state it was at before. The water parameters could be affected by your plumbing, or by your local water municipality if they have certain parameters they're expected to maintain.

Note: Even if you haven't had problems with your water in the past, it's possible your local water municipality could have changed or upgraded their equipment or methods, or simply changed their parameter requirements. That's why it's always a good idea to perform tests on your aquarium periodically to ensure that everything remains the same.

To test if that's the case, I'd suggest testing the water after it's been sitting in a bowl or a bucket for the amount of time it takes your tank's pH to change after performing a water change. Typically I think it takes a day or so before you start seeing anything noticeable.

pH is affected by a couple different things, but what I think is changing it in your case is probably the oxygen and carbon dioxide levels.

I know pH will rise if either carbon dioxide is taken out of the water or oxygen is added to it, and lower if carbon dioxide is added or oxygen is removed. So what I'm guessing that's what's happening to your water. Perhaps there are organisms in the water consuming oxygen and releasing carbon dioxide over time.

Some ideas on how to keep a higher pH in your aquarium:

The pH will typically lower over time eventually, that's part of the reason why water changes are important, to keep a stable pH level. But as your pH levels are changing more rapidly than what I'd consider normal, I think that it would be safe to consider adding things that will work to keep the pH levels up.

The most common method I think is using either crushed corals, or limestone. You can use them as a substrate, or place a bag filled with it into your filter.

The crushed corals would be calcium carbonate, which when mixed with water form calcium bicarbonate, which will absorb carbon dioxide in the water. Limestone is formed by the minerals calcite and aragonite which are different forms of calcium carbonate1, so it acts in the same way.

Note: This is different than carbon filters, which use activated charcoal to filter out chemicals and colors in the water.2

Otherwise something as simple as gravel substrate can help buffer the pH levels to become higher. Likewise, adding driftwood will help buffer pH levels to become lower.

Adding plants will help maintain pH levels as they absorb carbon dioxide and release oxygen into the water. Although if I remember correctly, parrotfish like to eat plants like candy, so it might not be viable in your situation.

If nothing else, there are chemical additives that help raise, and lower, pH levels. I've used API's pH up without any ill effects. Of course use it in moderation so that the pH isn't changing drastically.

1 https://www.wikiwand.com/en/Limestone

2 http://www.theaquariumwiki.com/Activated_Carbon

  • I also like the idea of using a coral/limestone substrate. I'm becoming a bit tired of the bare-bottom look and might be looking for a change anyway. As you noted, the pH would drop gradually even under the best circumstances, so it would be interesting to see if the substrate I chose would buffer that change to some degree. Commented May 6, 2015 at 17:33
  • @RobertCartaino It's a nice look, basically the live-sand used for saltwater tanks. I have plain, white, sand in my aquarium and aside from the algae that shows up in it, I like it more than the gravel.
    – Spidercat
    Commented May 6, 2015 at 17:45

I also have a 75 gallon (around 280 liters) tank that loses pH drastically. It is also a bare bottom which I did about a month and a half ago and has some plants. My water comes out of the tap at 7.6 pH and two days later it's 6 or less. After testing everything possible, my tap also has low KH and GH, which also lowers pH. I never had a problem with keeping a high pH for my goldies when I had a thick gravel substrate but removed it as my fish are now so large one actually had gotten four pieces stuck in his mouth and I had to pull them out. I just added a bag of crushed coral from amazon.com.

I put a smaller bag of it in each of my filters a few days ago with no raise in pH. So I added a larger bag inside the tank today hidden in a ship ornament. Will keep increasing the coral until I see some changes. The bare bottom thing has really throw me for a loop.

  • 2
    Welcome to Pets Stack Exchange :) I'm not sure if this is what you meant in your post or not but if you have low KH, the PH can change more easily. If your KH is high then you'll struggle to shift your PH. As I understand it, having a lower KH doesn't affect the PH value that you have, just that the PH can fluctuate more easily.
    – Henders
    Commented Oct 11, 2018 at 8:17

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