The last reply was a bit short as I had to leave quickly. Better explanations may help.
NAtural environments are difficult if not impossible to match in our enclosed tank systems. Water exchange, and fish per gallon ratio's are extremely different. The only real way to come close is to do massive daily water changes, or design a system that constantly changes water for you.
So the real truth about pH in an enclosed system.
As a general rule, pH and KH are directly correlated. If you alter one it will alter the other. So the only real way to lower pH is to remove KH. This is not a wise move in reality.
KH is the buffer in our tanks against rapid pH swings and deep dips. Our BActeria consume KH and essentially produce acid. So if we lower KH to begin with, and our bacteria begin consuming what is left we see pH drops (Sometimes quite rapid) and instability in the tank. So we need KH. My personal reccomendation is 2-3 dKH (1 Degree = 17.9 mg/L) and if you maintain KH at that level you will see a pH of about 7.4-7.8 depending on CO2 levels. Co2 is about the only thing that can change the pH/ KH correlation. This correlation is strong enough that we can actually calculate Co2 levels in our tanks by comparing KH to pH normal atmospheric co2 levels fall somewhere between 2 and 4 ppm depending on height above sea level and a few other factors.
If you maintain a lower Kh level you stand at risk for Kh depletion and the pH swings that it creates.
Now, the pH myth with Fish. First of all I'll just say that fish are adaptable, and can be acclimated to and kept hapily in just about any water within normal hardness ranges. Most softwater fish prefer low levels of dissolved solids. In natural waters the two predominant elements are Ca and Mg. Since natural calcium is mostly found in the form of CaCo3 (Calcium Carbonate) the genral accepted rule is that hard water has a high pH. Dissolve calcium carnbonate and you get an elevated GH as well as Kh and thus the pH rises.
In municiple water sources, these rules are often manipulated quite a bit. Additionally as a hobbyist you can manipulate them (If you really want to) So if you wanted ultra soft water for angels and Tetra's , You could start with a pure water source (RO or DI) and then add something such as Sodium Bicarbonate (Baking soda) to get a Kh of 3 dKh. This would give you very little dissolved solids, ultra soft water, and a pH of about 7.6. It would also give you a very stable soft water tank. I suspect that one could breed discus in water set up that way, but have not proven it yet. I know Tetra's will breed in some fairly hard water if other conditions are met.
So in a nutshell, Don't chase the mythical pH number of the wild amazon. Acclimate your fish to the tap water for ease, and if you want to breed fish (And they won't breed in your tap water) then get a good grasp on the chemistry, and create a stable soft water tank (Which will still have a pH high above the norm)
Discus Breeders who maintain Ultra soft water (Typically through the use of RO) also do massive daily water changes to ensure stability. If you deplete KH the only way to keep things stable is through constant removal of waste before the biological processes get too large. Most hobbyists do not need to or want to maintain tanks in this fashion.
Peat and driftwood Essentially soften water naturally by Leaching tannic acid which consumes (Not sure if that is the technically correct way to say it) Caclium Magnesium and carbonate. So if we use Peat or driftwood to soften our water, we are removing Kh and basically accellerating the process that leads to crashes and instability. With the correct mix of things, and a good maintenance routine, this works for some folks. but not all and not without some serious attention to detail. Essentially those who successfully use peat are doing water changes with decent levels of Kh, which is worked on by both the tannic acid and the bacteria, and the next water change occurs before they run clear out. I have even read of methods that involve Adding Crushed coral (CaCo3) and peat to the same system. one adds carbonate one removes it. Properly balance itwould work, but seems like too much work for me.
Crushed Coral, Aragonite, limestone, etc. are CaCo3. My experience is that these items all dissolve at a rate that largely depends on pH. Most forms of Calcium carbonate quitdissolving at about 7.6-7.8 pH. so if you have CaCo3 in your substrate, It will do everything it can to keep your pH at 7.6-7.8. The lower you try to take your pH the faster your sand will dissolve to counteract it. Of all the forms I've tried Aragonite seems to be the one that takes pH slightly higher and dissolves the quickest. I am fairly certain that carib sea sand is mostly aragonite (but read the label there is more than one product under that name) I'd see how it reacts but you may want to pick a different substrate.
Either way I would not at all be worried about putting angels and tetras in a tank with a pH of 7.8.
HTH
Dave
