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I went to the pet store to get some advice regarding the water quality in my 100L freshwater aquarium (which is stocked with fish).

Since taking over this aquarium, I've notice that the pH tests no more than 6.0. The colour of the test solution corresponds to the most acidic level covered by the chemical test, so the real pH could indeed be less than 6.0. The chemical test also shows ammonia and nitrite levels consistent with 0 ppm, but nitrate levels always test high, at about 40 - 80 ppm.

We've suffered a number of deaths in the aquarium recently and so any problems with the water quality is suspect number one.

The advice I received to sort out the pH level in the aquarium was to add Stability, treating the aquarium according to the instructions for Stability.

From what I understand, stability sets up the bacterial cycle in an aquarium, and has nothing to do with sorting out pH or providing a pH buffer. Am I wrong?

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Stability establishes bacterial colonies, meant to aid in the breakdown of ammonia, nitrites, and nitrates (source). Tanks that have a problem in them or are neglected will gain a lot of nitrates. This also causes the pH to go down. The point of Stability is to lower your nitrates and allow your pH to come back up. This is part of the solution, but there are others as well.

For now I would do a few things:

  1. Don't put any new fish in your tank until this is sorted out.

  2. Find out what the pH of your tank really is. This means getting your hands on a test that goes below 6. Common guppies can die at a pH as low as 4.75 (source). You want to know your actual pH because if you don't, then you won't know how to properly adjust it. Try the pet store or Amazon.com. You will want something that can read lower than 6.

  3. Find out what the ideal pH for your fish should be.

  4. Purchase the Stability or another cycling agent and also a 'pH Up' adjustment chemical. This can be bought at the pet store usually. This will only mask the problem for now, but it can be useful in an emergency. When you do a water change, add the recommended dose to the new water. Do not change the pH by more than 0.5 per day. This will stress out the fish.

  5. Test your kH. This is how stable your pH will be. You can also purchase buffer solutions that will stabilize your pH. You don't need to do this until you get a more desirable pH.

  6. Buy dead coral from the pet shop. This will naturally raise the pH over time. Keep in mind you may not need this once you have a stable pH. Rinse it thoroughly and soak it as well to remove any salt left behind.

  7. Test the pH of the water you are adding to the aquarium during water changes. This is useful to know.

  8. Perform a 20 percent water change today and every second day, adding the Stability regularly, as directed. Keep changing the water until your nitrates at least go under 20 ppm.

  9. Find some plants, preferably, ones with good roots. Plant them firmly once your pH is at 6. This will help bring down the nitrates slightly.

  10. Once your pH is ideal, make sure you add the appropriate amount of buffer solution. This is, again a short term fix, but still longer than adding ph Up every day.

  11. Promote that bacterial growth. Make sure you never change out the biological part of the filter so that you keep those bacteria.

At the end of the day, your pH will be buffered mainly by your coral (or driftwood if it goes too high), the bacteria colonies, and your kH hardness buffer. This could take up to 2 - 8 weeks depending on how bad the problem is.

Another person with a similar problem: link

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You are correct about stability, has nothing to do with pH whatsoever.

First of, you need to figure out what the actual pH of the tank is. Test strips are notoriously inaccurate. Even a cheap API test kit should be much better than a strip. If you have a good local fish shop, take a sample of water to them and see what they test it as. You also need to get an accurate measurement of the pH of your tap water. Test it straight out of the tap and let it sit overnight and test it again. The main concern here is if your tap water is significantly different than your tank water. If this happens to be the case it would be a good idea to try and figure out why there is such a large difference and make plans accordingly from there.

Unless your pH is extremely acidic or basic, it's best to just ignore it unless you are trying to breed a specific species. pH in a tank is variable mainly because of its correlation with CO2. Algae and plants will consume CO2 when they are illuminated which is why pH goes up during the day time. Have multiple people in a room and you can measure the pH drop in a tank. Opposite thing can be said if you open a window or turn fans on in the house.

Example, this is from my main reef tank.

enter image description here

You can see the ph swing throughout the day, but on the 21st, we turned the AC back on which reduced the CO2 concentration in the house and the overall pH went up with it. It's just such a moving target that it's better to not mess with it.

Don't try to buffer it and don't add buffering agents unless you know exactly what you are doing. At best these are a temporary solution as they aren't permanent and will greatly increase the maintenance on the tank as well as having to pre-buffer the water during a water change.

Most fish can thrive just fine in lower or higher pH than what is their ideal. I keep discus in pH 9 tap water and have never had a problem with it. The main thing that all fish need is stability.

You nitrates are fairly high. Based on the specs you listed, this is what I would assume is the problem. Also, if these fish were newly added or this is a new tank, it wouldn't be uncommon to have losses even if you properly cycled the tank. This is subjective, but I would start doing 50% water changes per week, always make sure to treat the water for chlorine or chloramines, and see where your nitrates and fish health are in a month or two. Reduce water changes once you get the nitrates in control.

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