I copied and pasted the below from an article on the effects of acidic rain on aquatic systems.
AFFECTS ON AQUATIC SYSTEMS
Mollusks - snails and clams.
- these invertebrates are highly sensitive to acidification because of their shells which are either calcite or aragonite (both forms a CaCO3) which they must take from the water.
- in Norway, no snails are found in lakes with a pH of less than 5.
- of 20 species of fingernail clams, only 6 were found in lakes with pH of less than 5.
Arthropods
- crustaceans are not found in water with a pH less than 5.
- crayfish are also uncommon in water where the pH is less than 5. This is an important consideration because crayfish are an important food source for many species of fish.
- many insects also become rare in waters with a pH less than 5.
Amphibians
- as you may know, many species of amphibians are declining. To what extent acid rain is contributing to this decline is not exactly known. However, one problem is that in places like northeastern North America amphibians breed in temporary pools which are fed by acidified spring meltwater. In general, eggs and juveniles are more sensitive to the affects of acidity.
Zooplankton in lakes
- changes in diversity among zooplankton have been noted in studies carried out in lakes in Ontario, Canada. These studies found that in lakes where the pH was greater than 5 the zooplankton communities exhibited diversities of 9 - 16 species with 3 - 4 being dominant. In lakes where the pH was less than 5, diversity had dropped to 1 - 7 species, with only 1 or 2 dominants.
Periphytic algae
- many acidified lakes exhibit a large increase in the abundance of periphytic algae (those that coat rocks, plants and other submerged objects). This increase has been attributed to the loss of heterotrophic activity in the lake (i.e., the loss of both microbial and invertebrate herbivores in the lake).
Fish
- as a result of acidification, fish communities have suffered significant changes in community composition attributed to high mortality, reproductive failure, reduced growth rate, skeletal deformities, and increased uptake of heavy metals.
Mortality
- effects on embryos and juveniles:
- Atlantic salmon fry have been observed to die when water with pH < 5 was introduced into breeding pools.
- in fish embryos, death appears to be due to corrosion of epidermal cells by the acid. Acidity also interferes with respiration and osmoregulation. In all fish at a pH of 4 to 5 the normal ion and acid/base balance is disturbed. Na+ uptake is inhibited in low pH waters with low salinity. Small fish are especially affected in this way because due to their greater ratio of body and gill surface area to overall body weight, the detrimental ion flux proceeds faster.
- in all fish low pH water causes extensive gill damage. Gill laminae erode, gill filaments swell, and edemas develop between the outer gill lamellar cells and the remaining tissue.
- at pH <3 coagulation of mucus on gill surfaces clogs the gills, which leads to anoxia and subsequent death.
Reproductive Failure
Reproductive failure has been suggested as the main reason for fish extinction due to acidity. In Ontario, Canada it was observed that in acidified lakes female fish did not release ova during mating season. When examined, the fish were found to have abnormally low serum calcium levels which appears to have disrupted their normal reproductive physiology.
Growth
Growth may increase or decrease depending on resistance of a species to acidity. For resistant species, growth can increase due to the loss of competing non-resistant species. On the other hand, growth can decrease due to increase in metabolic rate caused by sublethal acid stress. In this case the organism's rate of oxygen consumption goes up because the excess CO2 in the water increases the blood CO2 level which decreases the oxygen carrying capacity of the hemoglobin.
Skeletal Deformity
This occurs in some fish as a response to the lowered blood pH caused by increase in CO2 described above. Bones decalcify in response to a buildup of H2CO3 in the blood as the body attempts to maintain its normal serum osmotic concentration (i.e., the body attempts to return to a normal blood pH level).
The only thing I'd add personally is that a GH test tests for total calcium and magnesium levels including oxidized ones that are no longer useful to inhabitants and plants, etc.. which eventually happens if not replaced. Similar to why iron is not always available to plants because it's oxidized very fast by the oxidizers present in tank water. So a GH reading of any amount does not indicate absolutely anything useful. Eroding snail shells on another hand indicate the water is lacking non-oxidized calcium and is too high in oxidizers that extract the only available free electrons around left which happen to be from the calcium in the poor snail shells.
So maybe it's time to add some to the tank or do water changes with water that contains some.
I wouldn't worry about soft and hard water for fish and what the tests show, but rather what your inhabitants tell you by their health.
Similar to copper and shrimp. High amounts of copper kills shrimp, lack of any copper kills shrimp too as they use Hemocyanin for oxygen transport and immune system defence which is a copper containing protein.
