water parameters

Question
Hi,

I have a 20 gal. freshwater tank that I finished cycling about 3 weeks ago.  Over the past three weeks I have slowly added 4 platys and 6 blood-fin tetras.  I would like to add 2 or 3 albino corydoras at some point too.  I have zero ammonia and nitrites, and my nitrate levels are at about 15ppm.  I have been doing 25% water changes since adding the fish.  My tap water is soft at 89.5ppm and the alkalinity measures 60ppm.  However, the ph has been consistently 7.4 from the tap and in the tank.  I've heard it is somewhat unusual for water to be soft with relatively low alkalinity and also base in PH.  My question is: Is this alkalinity level too low and if so, do I have to worry about PH drops/shifts?  
Do I need to add a PH buffer? If so, which one do you recommend and how best to add it?  I would appreciate your help with this!!   Thank you!

Answer
Hi Grace,
Its best to leave your water just as it is compared to trying make the pH perfect with chemicals. This is because adjusting with chemicals runs the risk of uncontrollable pH swings and as long as your pH remains stable then I would leave it where it is. Your fish will thrive.

The 2-3 albino cory addition is a great idea. Sounds like a lovely tank so far!

I hope this helps and best of luck!
Karen~