Here is a direct quote from JimmyL, extracted from Discus As A Hobby web site. If this helps anyone that's great. If you want more information or pictures you will have to contact JimmyL. He appears to be very helpful. Bill
Quote
It's hard to say....Depending on where your Heckle's came from. I bred the Nuhmunda Heckels at Ph-4.5 and the Medeiras at 6.8 they varies in different regions. Blue face is harder to breed too. They all need low conductivity as low as 60 microseimen. Ph crash is very easy. I always use a central system with large volume 10 x 30gallon tanks in series with total capacity of 300 gallons to keep ph stable. I first isolate the breeder with no water change and let the Ph drop by itself until they have spawning activity and lock it there by join in the breeding tank with the central system at the same PH to keep it stable. You have to find your own range.
Jimmy