A few months ago, we took over ownership of a small fish tank (with fish) from their existing owner. They had kept fish for many years without trouble, and told us that they changed the water every 2-3 weeks, and the filter cartridge every 2 months.

I don't know what size the tank is precisely, but measuring water in and out suggests 25-30 liters. It's currently got 4 small (platy size) fish in it. We try and feed them only a minimal amount, as suggested by the previous owner.

We've discovered that the water becomes acidic very quickly after a water change. We treat tap water with an additive that makes it "safe for fish" and another that's supposed to retain pH balance. The water out of the tap is about pH 7, and remains pH 7 after these are added.

However within 2-3 days the pH has fallen to 5.5-6. I'm not sure if this is bad for the fish or not, but since owning the fish, two have died (there were six originally), although this could have been old age. One fish did develop what looked like a red sore spot but this has receded since we started changing the water more often.

Had I realized I'd need to changing the water more than once a week, I'm not sure I'd have agreed to having fish. Is this frequency of water changing typical of a tank this size, is the acidic pH acceptable for most fish, and if not, is there anything I can do to try and keep a healthy pH for longer?

  • Not posting as an answer because I don't know what to suggest, but it does sound unusual to me to be changing more than once a week even with a tank that size. Hopefully someone can suggest a reason it might be getting so low so fast. Apr 27, 2014 at 12:26
  • Since your tank was already established when you got it, you might start looking at your own routine too. Do you use an ammonia-based glass cleaner? Are you dumping tons of food in the tank?
    – user1259
    Apr 28, 2014 at 4:24
  • Is it a planted tank? Are you using CO2? What is the substrate? Also is there any filter media i.e peat moss etc? Apr 28, 2014 at 7:13
  • There's no real plants in the tank (they seem to die with astonishing speed), but there are plastic ones for the fish to hide in. There's no additional CO2 or O2. The substrate is small gravel. Not sure what you mean by filter media - the tank has an electric filter with an artificial cartridge which we change every 4-8 weeks.
    – Bob Tway
    Apr 28, 2014 at 10:52
  • 1
    Try using inexpensive bottled water for your next water change, and see if the PH drops like that. That will tell you if its an issue with your water (for ex, chloramine vs Chlorine), or possibly something related to the tank itself. Apr 28, 2014 at 18:29

4 Answers 4


John Cavan's advice is good. Some additional advice that I learned the hard way after many years of owning 80 and 300 litre freshwater tanks:

  • If you don't have a working understanding of pH, it is helpful to read up on it. A precise definition of pH is unnecessary; a good way to think of it is that pH measures the amount of free hydrogen atoms in the water. Pure water has one hydrogen per 10 000 000 water molecules; note there are 7 zeros. Slightly acidic water has ten times as much hydrogen, so one per 1 000 000 water. Note there are 6 zeros. Slightly basic water has ten times fewer hydrogens, so one per 100 000 000, note there are 8 zeros.

Water with a pH of 5.5 has one hydrogen per ~310 000 water molecules so it is ~31 times more acidic than pure water. (Because 31 x 310000 is about 10000000.) That's pretty acidic.

  • Small tanks are considerably harder to keep in pH balance than large tanks because small changes have a relatively larger effect. Your 30 litre tank will be a lot more finicky than a bigger tank.

  • Baking soda -- NaHCO3 -- is a great addition to a tank if you're having pH problems. It is a "buffer", meaning that it will not only bring low-pH tanks back towards neutral, but it also resists the water becoming too basic as well. Essentially it acts like a spring -- if you compress a spring it tries to get back to normal. If you stretch a string, it tries to get back to normal. No matter what you do, it tries to get back to normal.

  • Washing soda -- Na2CO3 -- will raise the pH of an overly acidic tank faster than baking soda will. Given what you know from my first point, you should be able to figure out why that is. (Incidentally, Na2CO3 is what NaHCO3 turns into when you bake it; it's what's left in the cake after the baking soda produces carbon dioxide.) Sodium hydroxide -- NaOH -- will also raise pH quickly. (The OH- combines with free hydrogen ions to form H2O and thereby lessens the amount of free hydrogen ions very quickly.) However, see the next point.

  • I recommend measuring pH with a chemical indicator. A sad story: I got an electronic pH meter with my tank and used it to check pH for many years. One day it read that the pH was low, so I put a little NaOH in the tank. The next day, same thing. And the next. And then I realized wait a minute, there is no process that should be making that much acid that quickly in the tank. The pH meter was in fact broken, the pH was very very high, not low at all, and I ended up bleaching some fish to death. Chemical indicators don't break.

  • Note that the chemistry of your tap water can change considerably over time depending on where you live and what local conditions are like. Where I grew up there were two main sources of water; a glacial moraine aquifer where the water was insanely hard (hardness means "lots of dissolved CaCO3" and makes pH very stable) and treated river water. People who lived in the part of the city on the aquifer had the same water chemistry all the time, but the river water changed chemically with the seasons. If you're not sure where the water is coming from and how stable it is, keep an eye on it. Hard water sources will be more resistant to pH change than soft water sources.

  • The other answers suggest oxygenation; indeed, dissolved CO2 is acidic, as you can tell if you measure the pH of soda water. It's not clear to me that additional dissolved O2 will change that, but it is certainly good for the fish. Note that pumps which bubble air through the tank will help, but the mechanism is different than you think. Most of the absorption of dissolved O2 will not be from the bubbles, which have relatively small surface area, but rather from the fact that the bubbles cause the water of the tank to slowly rotate in a cylinder, bringing more water from the bottom of the tank into contact with the air. It's the top of the tank that has lots of contact with the air.

  • Thanks for your advice. FYI I'm a former biochemist, so I have a pretty good appreciation of what pH indicates :) I've got a liquid tester which, annoyingly, only has a colour chart for pH of 6.5 upward, but the colour it goes suggests a pH lower than that. I got hold of a couple of testing strips which confirmed it. And our water here is infamously hard. Appliances rock up in no time.
    – Bob Tway
    Apr 28, 2014 at 10:47

The appropriate pH level of the water does vary a bit by fish, but too acidic can definitely kill them. Any change in pH levels that you want to introduce should be done gradually to avoid a shock to the fish, but some ways to go about this include:

  1. Partial water changes, about 25% on a frequent basis. You don't have to change out the water completely, but regular, partial changes can increase the pH without the burden of a full change all the time. Also, I don't think I would be doing complete full changes of the water, this is very hard on the fish, partial changes are much better.

  2. Use baking soda, yes the Arm & Hammer kind. Take a couple of cups of aquarium water, dissolve 1/8th teaspoon of baking soda in it and then add it to the water. Wait an hour or so and then measure pH. If it's increased a bit, say 0.2 or thereabouts, stop and do the same the next day. This will take a few days.

  3. Aerate the water. Basically, what you're doing is driving out the carbon dioxide for oxygen and that raises the pH. Water, over time, will lose the oxygen levels and increase the carbon dioxide levels, thus increasing acidity. Adding oxygen back in changes that. You can get air pumps for your tank at many pet stores for very little. $20-$25 can make life easier for you... :)

  4. Rocks and/or driftwood from your pet store can influence the pH levels of the tank (and make them more interesting too), though wood will tend to reduce the pH. Check for your species before you buy though.

As I said, you have to do this gradually. A sudden, very, large spike in the pH can be just as harmful. Take your time and do it slowly, that will help them much more.

  • Rocks and driftwood can definitely influence pH. However, driftwood will actually tend to make water more acidic so it should generally be avoided in cases where pH is already dropping unusually low.
    – symbol
    Apr 27, 2014 at 23:21
  • @symbol - Depends on the amount of tannins left in the wood, if there isn't any, you shouldn't see a pH drop. However, it can be helpful for keeping the pH balanced in the water. Good point, though, so I edited.
    – Joanne C
    Apr 27, 2014 at 23:27
  • Thanks for your suggestions. I'd heard you could add baking soda to buffer the pH, but also that fish disliked pH swings. But adding it bit by bit seems sensible.
    – Bob Tway
    Apr 28, 2014 at 10:49

The info you've been given in the previous answers is good.

Water that has a decent buffering capacity (generally lots of carbonates) will be more resistant to pH fluctuations than water that does not. (If your water is pH 7 straight out of the tap then it likely has little to no buffering capacity which could definitely account for your dramatic pH swings.)

High levels of carbon dioxide in the water will tend to make it more acidic, so forcing the CO2 out should help to bring the pH back up again.

Partial water changes are also important since your buffer/carbonates can get used up over time, which can result in the water suddenly becoming more acidic. These partial changes are also necessary to remove nitrates from the water which will build up over time (as a result of fish waste) and can be harmful to your fish if they get to very high levels.

There are a few more things that might be worth considering when trying to account for fish loses and pH swings.

If the filter media and/or substrate dried out when the tank was being moved then it's likely that most of the beneficial bacteria that live there and convert fish waste (ammonia) into less toxic forms (nitrites and, eventually, nitrates) will have died off. This could result in a fatal ammonia spike if fish waste builds up faster than the bacteria can re-establish themselves.

You said that you have been using some sort of pH "balancer". From what I've seen, these often cause more problems than they solve. Check to see exactly what it is and how it works. Products meant to adjust pH can often result in less stable water conditions. And if you're looking for something to stabilize pH rather than adjust it, plain old baking soda (as others have suggested) is probably going to do just as good a job -- if not better -- and be much, much cheaper.

As far as using decor items to adjust your pH goes... this can work, just make sure you add things one at a time and carefully monitor your pH after adding each item so that you're aware of any rapid changes. As John Cavan said, rapid shifts in water chemistry should be avoided if at all possible. Adding things like crushed coral or limestone will add buffering capacity and tend to raise pH. Adding driftwood will actually lower the pH so I would advise against adding any wood to your tank if you're having trouble with it going low already.

In general, for a tank that is not overstocked, I would find it very unusual to have to do more than a 25% water change once per week. If you're having to change more water than this in order to keep the chemistry stable, then it's worth examining your stock levels, decor/hardscaping, water source, buffering capacity, water additives, etc. to see if you can identify the source of the instability. Hopefully the info you've been given in the answers here will be enough to pin it down and address it.

  • The substrate hasn't been allowed to dry out in a very long time AFAIK. When we got the tank, it had a few inches of water in the bottom (with the fish in it).
    – Bob Tway
    Apr 28, 2014 at 10:50

If you're experiencing sudden drops in PH after a water change, run an experiment by filling a bucket with tap water. Test the PH immediately after filling and then again the next 24 and 48 hours. If it drops then it's likely due to the release of oxygen and absorption of carbon dioxide from the normally fairly benign tap water after it has rushed out of your tap. You can let the buckets settle and adjust PH with a stabiliser if you wish, but also experiment by using a submerged hose to fill the buckets to reduce surface agitation. Again, check PH immediately then after 24 and 48 hours. If there's little to no change using the submerged hose technique you have a winner for water changes.

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