The best PH level for cichlids is 6.8-7.2. In discuss cichlids, this level will be much lower (pleas note- discuss are VERY hard to keep).
6.5 to 8.3___________________ Oscars can be kept as said above in a wide range of PH, however they do best in a neutral PH as do most other South American Cichlids which Oscars are. (Neutral PH is 7.0)
It depends on the type of fish and plants that are in the aquarium. For most tetras, danios and swordplants, a pH of 6 is preferred. For guppies, platies, swordtails, hornworts and cichlids, a pH of 8 is ideal.
No, cichlids require lots of space, and they are very aggressive towards other tank mates. If you want cichlids, get a large tank and only put other cichlids in it.
I have a 260 L tank and have to rainbow sharks with 12 assorted cichlids and have not had any issues.
yes
Red cichlids I am assuming as red devil cichlids and in which case at least a 75 gallon as they get upwards of 13 inches. Also, additional fish in the tank would diminish any aggressive behavior so they would not be focused on one another.
the best pH for African cichlids is anywhere from 7.5 to 8.5 they like a high blue colour if you have a pH tester.
If a fish tank needs a pH of 5, but at present it is at 7, more H atoms should be added to the concentration.
You should NEVER try to mix African Cichlids with Amazonian Cichlids. It simply can not work. If your tank is set up properly for Africans your water conditions will be slowly killing the Oscar. Please for the Oscars sake get it out of there and in future do some homework/research before mixing species together.
other cichlids or other aggressive fish
99% of the African cichlids are very aggressive fish and will more than likely cause the untimely death of anyother type of fish (and usually same type) that you place within the confine of your aquarium. Also, one should note that PH levels differ between the two. While you can acclimate up and down with fish (Most African cichlids locally are kept PH 7.0 instead of recommended PH 8.2) not all will take the change in stride.
I had peacocks with my yellow lab mbuna and white top hera mbuna cichlid. they will tussle with each other periodically, but it should be fine so long as you have than 4 or 5 to help spread aggression.