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Fish don't need stable pH. They do need stable TDS (total dissolved solids) in the water. If you have plants, and get enough CO2 into the water for their needs, your pH will be around 6.5 every day with the CO2 and lights on, but will drift back towards 7.5 at night, with the CO2 and lights off. That kind of pH changes doesn't bother the fish.
 

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In a fish only tank pH is a good measure of the "quality" of the water. If the pH swings, it is because something is wrong in the water. pH is easy to measure, compared to most other parameters, so it became the rule that fish need a steady pH.

In a planted tank, pH has other meanings. The plants act to stabilize the water conditions. If the pH changes in a planted tank the first most likely reason is that the concentration of CO2 has changed. The next is that some carbonate is dissolving into the water, or pieces of wood are releasing tannins. Those three things, when they occur relatively slowly are not bad for fish.

The fish said to need low pH really need soft water, with low TDS. Those that need high pH need harder water, with higher TDS.

Any time you add something that changes the water conditions, as peat does, you set up a need to match that water condition every time you do a water change, which can be such a drag that you just delay and delay water changes. But, those water changes are very beneficial to the fish and the plants.
 
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