I used to tell people when I worked in a pet shop as a side thing for a while that you can keep and breed a young pair of convicts in the toilet if you wanted... I wouldent do it, but they are that hardy.
as others have said, constant stead ph is much more important (especially for a fish so far from the wild as convicts) than trying to manipulate the ph and take the chance of ph swings. Besides, I try to avoid adding any type of chemicals when at all possible.