I have used crushed coral in my pressurized co2 tank. That was until I learned a lot more about water chemistry and the nitrifying bacteria. It is not true that there is no nitrificiation in acid water. It is true that it may not be the exact same strains of bacteria at work. There are two things going on here. The first is that the bacteria have receptors for ammonia (NH3) and ammonium (NH4). Not all the bacteria have both. What happens as one drops the pH level in a tank is the bacteria at work may change some but there are still some working. But the bacteria do not process NH4 as effectively as NH3. Fortunately, NH4 is way less toxic. I have kept wild Altum angels at 6.0 and have for a few years. I brought them into a tank with a pH of 4.2 and worked them up to their present level over 6 months. I actually worked a plan to get filters cycled at 6.0 simultaneaously. The goal was to add the filters to the "uncycled" angel tank when it hit 6.0. By the time that happened, the angel tank had cycled itself.
Next, co2 and pH in water are not simple things to grasp. That is because CO2 is a gas which dissolves in water. A very small percent turns to acid, But this is very unstable and tends to vanish as fast as it is created, However, it is instantly replaced. This is why CO2 in water drops the pH. It is also why the more cabonates and bicarbonates in water (these are what makes up KH) the more CO2 that must be added to change pH. "In the piece below I have highlighted the carbonic acid in red.)
Carbon Dioxide
Carbon dioxide behaves very differently from the other gases in air. When it dissolves in water, it reacts to set up a series of equilibria:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
A small fraction of the CO2 that dissolves in water reacts rapidly to form carbonic acid. This, in turn, partially dissociates to form hydrogen, bicarbonate and carbonate ions. CO2 will continue to dissolve until equilibrium is reached.
The situation is further complicated by the presence of carbonate and bicarbonate ions in hard natural waters. These concentrations can be relatively high, resulting in high levels of carbonic acid and dissolved CO2. In these circumstances, dissolved CO2 concentrations can be 60ppm or more.
Like other gases, CO2 will pass unaltered through filters and UV treatment, however, due to its equilibrium with bicarbonate and carbonate ions, it can, in effect, be retained on anion exchange resins. As the bicarbonate and carbonate ions are removed on the resin, more CO2 will convert to bicarbonate until none remains.
from
https://www.elgalabwater.com/blog/dissolved-gases-purified-water
Basically, the potential co2 concentration in a given bofy of wayer are very dependant on the KH levels in that water. For this reason it is not possible to state a given concentraion of co2 can be achieved in water unless this has taken into account more factors. One other thing to note is if one is p[adding co2 to their water and they are seeing the KH level drop, all you have to do is stop the addition of co2 and the KH will come back up to its normal level. This is why acid will make rocks with a high calcium conten fizzwhen one puts acid on them.
As fishkeepers we actually know very little about what is in our water. Sure we may measure ammonia/nitrite/nitrate and we can test GH and KH. However, these last two tests are not telling exactly what is contributing to the numbers they provide. By that I mean if we know our tank water has a GH of 5 dg (or somewhere in the range of about 90 to 108 ppm), we have no clue what are the components of that reading.
Next. harking back to my Altum angel project. I quickly learned that the hobby test kits are pretty much useless when it vcomes to testing acid water. They tend to stop at 6.0 on the downside. So if you had water that was pH 2.0 it would read 6.0 on the test kit. So would 3.0, 4.0 etc. This means when one gets a reading of 6.0 pH on a test kit, all this tells us is that 6.0 is the highest the pH can be but not how low it might actually be. That is wgy I had to buy some priced digital test equipment in order to be able to keep the Altums.
As for adding crushed coral, this is also a quantity issue. Folks who have softer lower pH water who want to keep rift lake cichlids tend to use crushed coral or aragonite as their substrate. But the goal for such tanks is to raise both the hardness and the pH.
Hardening Your Water (Raising GH and/or KH)
The following measurements are approximate; use a test kit to verify you've achieved the intended results. Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.
To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter.
To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.
from
http://fins.actwin.com/aquariafaq.html
(Bear in mind that many fw fish do not like much sodium in their water, so I rarely suggest this as a way for one to raise KH except during a fishless cycle.)
Finally, when talking about adding crushed coral to help hold up KH, this is not the same thing as using it in quantity as a substrate. I ran about 1 cup of coral in a bag in the filter on a 50 gal. tank with pressurized co2 added. This amount did not change my GH, this would have required much more coral. The coral dissolves slowly and one will normally need a number of days for it to reach its maximum effect. It is also disappearing and it will need to be replenished with time. I ultimately gave this up as I ran a fairly lean level of added co2 and rarely exceeded 1 bbm. This also allowed me not to have to deal with the potential downside to allowing the co2 to run 24/7. Please note, this all worked for me in my specific set-up and tap water parameters. Water chemistry is complex and every tank is somewhat unique. It usually takes time to get things right. Therefore, it pays to go slowly until we are sure what effects any changes we make might have,
