My dad used to keep Oscars and he never had any problems with gh and ph. Does that depend on the different type of fish?
I had this problem prior to all of the ick problems and I'm not sure why because when I kept goldfish, I never noticed any ph problems. Are livebearers just more sensitive to ph and gh?
Yes to your question "does that depend on the different type of fish?" and here's why.
The oceans are all the same with respect to hardness (GH) and pH. So all marine fish are physiologically designed to function in such water. Fresh water is very different, and I will try to explain.
As water evaporates from the oceans and condenses into clouds, it loses all the minerals and becomes what we may term pure water. Water in its pure form does not exist in nature [by which I mean water on the surface of the earth as in watercourses, lakes, groundwater]; it is a powerful solvent, meaning a substance that easily dissolves other substances to create a solution. As rain falls, it picks up many gasses and particulate matter, and it continues to do this as it passes through the ground. Natural water values therefore vary with respect to hardness and pH because the water acquires specific properties from the landscape. Water flowing over or through rock will assimilate minerals from the rock, becoming what we term “hard” water. Water flowing through soils that contain organic matter will be “soft” because the organics bind with and thus remove minerals while creating acids that enter the water. The pH is largely the result of the hardness as well as the amount of carbon dioxide dissolved in the water.
As each freshwater fish species has evolved over thousands of years, their physiology has adjusted to the water values that occur in their respective habitat. We refer to these values as water parameters, and they include hardness, pH and temperature; each of these has an impact on fish. While many fish species appear to be somewhat adaptable, their physiology can be negatively affected if the parameters are outside the fish’s natural preference. Providing suitable water parameters in the aquarium is therefore an important aspect of providing an environment that is less stressful—and this directly relates to healthier fish.
Pure water has a pH of 7.0 which is neutral. A pH above 7.0 is termed basic (used to be alkaline but that is a confusing term) while a pH below 7.0 is acidic. The water becomes more basic the higher the pH is above 7, and more acidic the lower the pH below 7. No water in nature has a neutral pH, it is either acidic or basic to some degree because of the above described solvency.
Livebearers occur in Central America and Mexico. The fresh water throughout this area, generally speaking, is moderately hard with a basic pH. The fish are thus designed to function in such water, and they assimilate the minerals (primarily calcium and magnesium) from their surrounding water directly into the bloodstream. So the fish must have moderately hard water in order to function, and if they do not they will slowly weaken and die. Along the way they may contract various health issues that they should normally be able to deal with, except the lack of mineral prevents their metabolism and physiology from functioning properly as nature intended.
Soft water fish also assimilate water into their bloodstream continually. This water must be soft. If the water is on the hard side, the minerals again will be assimilated into the fish, but now they will start causing internal problems for the fish, and here again the fish will weaken and may or may not die, depending upon the species and the level of hardness. One detriment which is proven by tests is calcium blockage of the kidneys of soft water species, and studies in the 1980's with cardinal tetras determined that the fish's lifespan was directly related to the GH of the water when it was harder than what the species prefers. The higher the GH, meaning more mineral in the tank water, the faster the calcium built up and killed the fish. In very soft and acidic water, cardinal tetras can easily live beyond 10 years, but they never reach this age in water that is harder, and the age is relative to the GH and somewhat pH also.
You mentioned Oscars; this species has a preferred GH range of soft to moderately hard (say 5 to 19 dGH), with a pH range of slightly acidic to slightly basic (say 6.0 to 8.0). Livebearers need moderately hard or harder GH (10 dGH absolute minimum, and higher) with a basic (above 7.0) pH. Your water was assumed earlier in this thread to be on the soft side, with an acidic pH. So Oscars will probably be fine, but livebearers will never manage and will slowly (or sometimes more rapidly) weaken and die. The weakening makes them more susceptible to other health issues like parasites, bacteria, fungus, etc, so the actual death may be due to one of these, but in fact the actual cause is the water parameters.
It is much easier to select fish species suited to your source water's parameters, rather than trying to adjust the parameters. It can be done, but there are risks along the way, expense, and issues preparing water in advance of water changes, etc.
Hope this helps in understanding the significance of water parameters for fish. Because of the physiology of fish, this is significantly more crucial than the comparable environmental issues for any terrestrial animal, generally speaking. The aquatic environment and the life processes of fish is interconnected in ways that are so great it is difficult to actually comprehend them, but they are crucial.