[Peter Finke]

It is wrong to say “decay is dangerous”, but it is right to say “decay could be dangerous”.
I cannot comment on Helene’s problem; I don’t know what the cause of the black zones in the bodies of her fish are. But too much decay of rotting organic material can lead to such phenomena.
And certainly it can lead to severe hygienic problems.
This is one of the problems of blackwater aquaria, especially if they are small: You have to use much organic material (wood, leaves) which will not stay stable but undergo a process of continuing decay. Therefore, one has sharply to distinguish between fresh organic material and decaying organic material. And you have to consider the volume of a tank. The smaller your tanks are, it is certainly the more important to consider that problem.
If you begin a new small Parosphromenus-tank with leaves and wood, you should take fresh dead leaves and not too old wood. For instance, I take one sea-almond-tree leave and a handful of beech-leaves and some dead wood from a pet bog in a fresh set-up 10-liter-tank. This will have a good influence on it’s hygienic status for about at least four, maybe eight weeks. The sea-almond-tree-leaves are the first to rot, and then the positive influence turns into a negative. If it becomes smeary and greasy (and it will after some time), you should take it out and put a fresh in instead. The same happens with the beech leaves, but their decay takes more time, maybe months. Finally they have to be changed for new, too. It is very difficult to assess the decaying rate of wood, since there are very different forms and states of wood. Fresh dead wood from a peat bog is best, it will help you to create a stable blackwater milieu for months, maybe a year or even two. But the time will come when you have to throw it out and take a new one.
I admit that often I do not act in accordance with those rules. I had never that black spot problems, but I often had other problems (mostly fish disease, e.g. Oodinium and algae), telling me that some of my organic material has become too old and is rotting in a too-small tank. Then it is high time for a change.
Therefore, because we work with materials that are not normed but of very different natural origin, it is impossible to give a clear rule by quantitative measures. But one has certainly to consider the qualitative relations. Every dead organic material rots, but in different amounts of time. And all rotting is a load for a (small) tank. The beginning of that rotting process will have a positive influence on the blackwater milieu. But one has to learn when the point of a turnover has come; and act.