Can acidic water fish live in alkaline water?

For example, my tank pH is around 7.5 because its substrate is made of dolomite. I basically have molly and platys, but I want to have other kind of fish.

For example, I'd like tetra (neon) cardinal, or some big fish from Amazonian Bay (I can get they easily here in Brazil). These fish pH will be around 6 and 6.8.

I think I can put some tree trunk in order to turn the water more acidic. In the last case, I should change substrate (more painful).

Will acidic water fish die in alkaline water, if so, why?

  • 1
    I recommend to ask a separate question how to change the PH value, i.e. whether it's necessary to remove the substrate. Jan 17, 2017 at 4:01

2 Answers 2


It depends on the fish and depends on the pH. Normally I would say that stability is more important than a specific parameter range and I would still stick to that. I don't think many are going to have a hard time adjusting to a close to neutral pH like 7.5.

However, you're talking about wild fish which do not always tolerate drastically changed conditions very well. I do think any should be fine in you water as far as pH goes, but depending on the species, you may need to provide incredibly consistent and clean water for them to thrive, sometimes this requires changing 100% or more of the aquarium water per day, with treated and seasoned water, which means it has been sitting long enough that the pH and other parameters are stable. pH out of the tap will usually change due to degassing and settling over the first 24 or so hours.

"The Amazon" holds an incredibly vast diversity of life, and as far as species go, there's something like 50% of the species of fish on earth in that river. So it really depends on the fish you're looking to keep. Some specific Amazonian fish such as wild discus, altum angelfish and cardinal tetras do not tolerate anything but the cleanest and most stable water imaginable, otherwise they deteriorate or become sterile. For fish like these and many other sensitive ones, stability and the cleanest water possible is going to be the chore in trying to keep them. So, it's not pH you're going to have to worry about, but keeping up with water quality is going to be a near full time task depending on the fish.

  • Thank you for that information. I didn't know that stuff. I think I'm not buying new fish then because I don't have condition to provide such quality of water now, and I don't want to mistreat the fish. At least my mollys are giving birth and look happy.
    – Murilo
    Jan 17, 2017 at 13:07
  • There are plenty of fish in the amazon that are much easier to care for, just the ones I listed, and I'm sure there's more, can be difficult. There are many species of tetras, plecos, cichlids like non-blue or gold geophagus, and other common and uncommon fish that will not have the same stringent water quality requirements. Even non-altum wild angels like peruvian or other scalare are going to be reasonable easy to keep. I guess the quickest test it to stay away from fish in upper blackwater tributaties, as these tend to be much more sensitive.
    – Jestep
    Jan 17, 2017 at 16:45

You can orientate at the pH value recommendations for the fish you'd like to have in your aquarium. There's no strict border between acidic and alkaline liquids/water for the well-being of a fish. Values are provided by various sources, like theaquariumwiki.com or literature and if a value is missing you can ask here. You'll notice that most of them have a pH value tolerance reaching from below and above 7.

Note: The wrong pH value will only kill a fish if the value severely differs from the optimum, you should not only care about the survival of the fish, but it's well-being as well. Long-time damage, loss of fertility and unexpected behaviour are difficult to track down to changes to the pH value and animal abuse, so monitor the value well (meaning frequently and with limited measuring mistakes) and inform yourself well about how to change the pH value (your idea is good).

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