How do I keep the ph low for Discus?

You can get peat to put in your filter - this softens water and will bring your pH down. Also, adding some bogwood will have a similar affect. You can get blackwater extract as well. All of these elements will probably turn your water slightly tea coloured, but fish from naturally blackwater regions actually benefit from this - even if your don't like the look. I wouldn't mess around with chemical pH alterers as they can be very unstable. Generally, a stable pH is more important than a perfect pH and your fish will most likely have been brought up in harder water so should be fine outside of their optimal pH level.

Research using peat filtration and/or RO water. Altering pH to the point it stays stable is tricky and mind boggling! Locally bred Discus however can actually do well at a pH of 7, it's when you get into keeping wild caught Discus or Discus bred at those very acidic levels you have problems. They are much hardier to higher pHs these days than they used to be!

See here:

http://www.simplydiscus.com/library/water_chemistr...