to be honest..not much, but a pair of dwarf cichlids might be nice..
Apissto. agassizi perhaps.. or if you like less work,
kribs (Pelvicachromis pulcher), or its colourful cousin,
pelv. taeniatus.
If you regularly do maintenance, then with a bit of time you should be able to add a pair of otocinclus catfish,
If you don't fancy cichlids, theres always a shoal of tetras, or danios - some of them can be eye catching.
A male betta, and some smaller fish (neon tetra, zebra danio) might look nice too, but not sure on compatibility there..
PH is a measure of the acidity of the water. 7.0 is neutral, anything lower is acidic, which generally favours south American fish, and anything higher is alkaline, which generally favours african fish, there is exceptions to both of these cases tho.
Be warned tho, that acidity on this scale is logarithmic, as oppose to the normal linear scale. so PH 6 is not 1 unit stronger that PH 7, it's actually 10 times as acidic as PH 7. like wise, PH 5 is 10 times stronger than PH6, but, PH 5 is 100 times stronger that PH 7. In the alakine case, PH 8 is 1\10 of PH7, PH 9 1\10th of PH 8, and like wise, PH 9 is 1\100th the strength of PH 7.
Hardness, or water hardness, is down to calcium levels. easiest way to test this, do you have to scrub, hard and use lots of soap to get a lather? or can you fumble your hands around the bar a few times, and wash loosely, and get a lather? If your water is hard to get a lather in, its hard, if its easy, it's soft. There is also test kits to get an accurate reading. You would be better with an accurate reading, it help you make a decision on fish. Find something that lives in your tap water conditions, and you will save a fortune on water treatments, because the only thing you will need to add is de-chlorinator
