Ted, the two slim species ornaticauda and parvulus still present many problems to us. Especially the disappearance of many eggs over night is an unsolved problem.
I had some success by following an advice by friend Horst Linke to lower the pH to at least 4.0 or even 3.5. I know of the experience of Günter Kopic who had success with parvulus only with a pH around 3.0 (!). But equally, I had success with other pairs with a pH of 6.0 or even 6.5. It is for sure, that the activity of funghi and germs is much lower in a deeply acid milieu. That may play a role, but we do not know it for sure what other factors are of importance.