Is low pH in tanks due to aquasoils/softwater a concern?

2Hr Farm tanks typically have a pH of 5.0 when the CO2 is turned on.

Why do certain planted tanks have such low pH?

Aquariums with Aquasoil in soft water countries naturally have very low pH values. This is by design, as most aquatic plants prefer slightly acidic substrates and the vast majority of fish in the aquarium trade come from soft water, low pH rivers such as those found in the Amazon jungle. Peat is included in most aquasoils; it lowers the acidity of the water and also reduces the KH levels in the tank. For many brands of aquasoil, this buffering capacity can reduce KH to unmeasurable levels; this correspondingly lowers the pH, often to the 6+ range (which then drops to the 5+ range when CO2 is switched on, as dissolved CO2 is acidic).

Why is low pH associated with "tanks crashing" 

Very low pH/KH levels cause some concern among aquarists who fear that such a low pH will affect the health of the animals or bacteria in the tank, as one group of (usually older) aquarists have been taught that dropping/low pH levels in a tank are a sign that something is wrong.

There is some truth to this. In an aquarium, especially one that does not have regular water changes, poor maintenance can cause pH levels to drop. Bacterial oxidation of ammonia to nitrates consumes carbonates (which lowers KH/pH); although this effect is small, it can be significant over long periods of time. Organic waste by-products are also acidic, again the effect is small but the accumulation over time can be significant. The combination of these two factors can cause the KH/pH levels of a poorly maintained tank to drop over time. However, it is the accumulated organic waste that causes the problem, not the low pH itself, which is merely a by-product. Aquarists are therefore mistaking correlation for causation when they blame low pH for problems in a tank that has been poorly maintained over a long period of time.

Poorly maintained tanks can be expected to lose pH/KH over time. However, this does not mean that tanks with lowered pH/KH (due to the use of peat/aquasoil/CO2 etc) have poor water quality.

How we know that low pH systems can function well

There are many, many rivers in the world where the pH is naturally low. These include measurements in the Amazon basin, where areas of the Tapjos basin have a pH of 4.39, and the Tefe River has a pH of 5.03. The Tasek Bera River in Malaysia measures 5.33. This is a basket of spot readings, but these rivers are not outliers, such readings are common in soft water regions. All these rivers are also less than 1 dKH. These rivers are teeming with plants and fish - they are not wastelands where ammonia oxidation does not take place because "bacteria cannot survive below X pH".

Wild Toninas (above) in pH 3.2 black water swamps, coupled with Eleocharis species (below). Photos courtesy of Vin kutty.

Doesn't low pH harm bacteria?

Claims that bacteria function better at X pH is true for lab/petri dish conditions. However, such conditions do not reflect microbial functions in an ecosystem. The paper "Nitrification in a Biofilm at Low pH Values: Role of In Situ Microenvironments and Acid Tolerance" explains how nitrifying bacteria continues to perform their functions in low pH environments. 

Again, this is practically demonstrated by the great many examples of thriving wild habitats and planted tanks with low pH/KH values out there.

Should I buffer my aquasoil tank to raise the pH/KH?

No, you should not if you are keeping soft water animals / plants. Plants do not use KH and most plants actually function better at low ranges, as do most soft water fish. Many buffers also use phosphate based chemicals which will raise your PO4 levels significantly. Working against aquasoils is counterproductive.

If you are keeping animals that require hard water, then aquasoils that buffer down KH/pH should not be used in the first place. For tanks containing animals that prefer hard water, aquasoils should be avoided and inert or limestone based substrates can be used in the tank instead. Many African rift-lake cichlids, some species of snails and shrimps prefer hard water and their tank should be set up to provide this environment.

GH is separate from KH. GH comprises mostly of Ca and Mg ions, which plants use as nutrients for growth. To read more on GH head here

To read more on pH swings how they affect/not affect livestock head here

Tank examples from all around the world

As it is, we have a collection of examples of low pH/KH tanks from all four corners of the world. From a practical point of view, if this approach can be repeated with great success on four continents, hobbyists can be sure it will work for them.

Tom barr's tank has a pH of 5.6 when CO2 injection is on.

Gregg Zydeck's tank has a pH of 5.4 when CO2 is on, KH1.

Steve Tousignant has a KH of 0.

Paulina Zagorowska has a KH of 0

Hugo Alves tank has a pH of 5.8.

Daniel Constantin's tank has a KH of 0.

Rory Funch's tank has a KH of 1.

Joe Harvey's tank has a KH of 1.

Adam Wolferman Sr.'s tank has a KH of 1.