PH problem in angle tank,help!

[kuatto]

Got a call from an office with a fresh water tank.They are having problems with ph being high,fish are looking stressed and the plants are turning yellow(I haven't seen the tank).They are using one of those blue kits to test the water but need to lower the ph(she didn't say what the levels are). What should she use to lower the ph(if that is the problem)
I will get her to give me the levels and the brand name of the kit when she goes back to the office.