Eu tive um problema semelhante com o meu feto de java.
Comprei-o num tronco de ele mede cerca de 30cm de altura, ou seja, completamente adulto.
Como tive problemas com algas, começei a "brincar" com os fertilizantes e começei a suspender a fertilização. Resultado ao fim duma semana as folhas do feto estavam a ficar castanhas e a desfazer-se, ficando só as fibras e as nervuras.
Fartei-me de procurar sobre o que se estava a passar e encontrei este artigo. Quando o li fez sentido, retomei a fertilização. Remédio santo.
Sérgio Costa
Artigo:
"...but suddenly you may find Java fern melt beginning to appear.
Java fern melt is a rather suddenly appearing and protracted condition of deterioration of the leaves: The leaves develop spots and become mushy in places, then whole leaves rot.
If you have inadvertently allowed the nutrient levels to get too low. Although Java fern is a slow-growing plant, it still needs a modicum of nutrients. Boosting the nutrients brings the Java fern back to life.
Yet one can remove the leaves, and the rhizome remains viable. The Amazons, however, continued to look fine.
But this raises the question of why the swords, which are typically fast-growing and nutrient-ravenous plants, would continue to appear healthy while the Java fern, which is a slow-growing plant, deteriorates.
It seems that the swords, being rooted plants, can obtain nutrients from the substrate if the substrate is mature and rich, even if there are few nutrients in the water column.
The Java fern, being a plant that attaches to wood and rocks with a rhizome, cannot benefit from nutrients in the substrate.
So, when the nutrients become too few in the water, the slower-growing plant shows deficiencies, because it is not rooted.
Another condition that will cause Java fern to melt is a proliferation of blue-green algae, which are bluish-green "algae" that grow like film (actually they are cyanobacteria, bacteria that photosynthesize like algae)."
in http://www.pataquatic.com/pat-control.htm