Try and keep your nitrate levels below 40 ppm. If you are keeping Oscars and the South American fish them an ideal pH would be anywhere between 6.0 and 7.0. Having said that, a lot of people have high pH so Oscars are okay up to 8.0. I would forget about calcium carbonate for the time being, it can get very complicated and not many people understand there are enough to actually warrant worrying about it. Ph is a very complex subject, if you don't have a good knowledge of how it works and you must not try and altar at pH and other elements relating to it. You've got to remember that pH changes, if you've got 7.5 coming out of your tap, it won't stay like that once the fish had been living in it for a while. Obviously every time you do a water change, your pH will change. My advice would be to test your pH coming out of your tap, and then keep testing it as your aquarium gets established. You will then get to know exactly how your pH alters as you maintain your tank.
Phosphates are basically what causes algae. Phosphates won't directly harm your fish at high levels, although they can cause algae which in turn causes oxygen depletion. For this reason, regular water changes keep the nitrate levels low will keep them under control. I would say that keeping phosphates below 1.0 ppm will help stop algae growing out of control
