Using distilled water to lower gh & kh?

[TwoTankAmin]

I many cases merely topping off FW tanks will ultimatley result in disaster. As you are contimually hardening the water it stesses he fish. But even if they can adapt t enought ot survive, you make it extremely difficult to do a big water change if the need arrives.

And your numbers, as noted do not add up. Here is why:

Buffering Capacity (KH, Alkalinity)
Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The ``sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.

In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness'' (KH), ``alkalinity'' and ``buffering capacity'' are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity'' should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH > 7).

How much buffering does your tank need? Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. A tank's KH should be high enough to prevent large pH swings in your tank over time........

from https://fins.actwin.com/aquariafaq.html

I ran a Tank for Altum Angels for close to a decade. My tap was pH 7.0, kh 3-4 dg and GH about 5 dg. What I kept them in was pH 6.0 (it rended to rise a few .10s between water changes. I prefered to use TDS rather than jys KH and GH for testing and the tap ws 84 ppm and I tergetted the tank to be at 50-60 ppm. I Have an RO/DI unit which I used to make pure water for mixing with my tap for water changes. I also have a continuous digital monitor on the tank to give constant reading for hardness (either conductiibity or TDS), temperature (either F or C) and pH. As the tank parameters would change some between water changes I rou batch chaning water in a big rubbermade can next to the tank so I could transfer the probes from the tank to the new ater so I could set it's parameters according to what I need to get the tank's paramters back closer to into my targeted one's.

This was also a stained water tank. I used Alder cones, Almond leaves and Rooibos tea to stain the water and to help keep the water softer and pH at the lower target. Some weeks I would use some muriatic acid to get the changing alter to the need pH.

I waited until I had been in the hobby over a decade before felt I had sufficient experience and education to consider doing the above.

I also bred a number of the B&W Hypancistrus plecos from the Big bend of the Rio Xingu. Sometimes this required doing a dry and rainy swason simulation to try and trigger spawning. Over 3 months I had to ramp up the dry season- hardening the water and warming it up to over 90F. The onset of the rainy season dropped the temp by 20 degrees F in 26 - 48 hours and dropping the TDS (which had been raised some as well) in half in the same time period. But this was merely doing what nature does in the wild for these fish. Fortunately i only needed my digitial TDS tester for this, I could use my test kits for pH and is needed to check GH and pH if I needed to dig into some of the specific components of TDS.

While i was able to drop the pH for Altums by one full point in under 5 minutes with no ill effect, the drastic change for the plecos was more hardness/TDS and temperature oriented.

Changing ones parameters, especially lowering them, is a tricky business and contrlloing oit usually requires some expensive testing equipmment beuond what the standard hobby kits offer. Because I was staining the water for the Altums, test kits using colors to show the result were useless as the water was always stained a reddish brown.

 

[carlosbenjamin]

Yeah, swapping with distilled water will gradually lower GH and KH, but it’ll take time. Your plan should work, just monitor levels to avoid sudden swings. You might also consider adding driftwood or peat to help soften the water naturally.