As faor PH, hardness & alkalinity... they aren't very picky... just keep it stable and they'll be fine in those ranges...
Ammonia and Nitrite should be kept at zero... which is easy to do with a mature tank, new set ups have to cycle and are likely to have "mini spikes" when new fish are added. If that doesn't make sense do a search on the word "cycle" and you'll learn a ton.
Nitrates at 20 are awesome... They are best kept 'in check' with weekly water changes... everyone must define what 'in check' means for themselves... personally I try to keep all my tanks under 40ppm... I don't worry as long as they stay under 60ppm... even when life is kicking my butt and I'm slacking off I won't let them ever get over 100ppm...