I have a soft water tank with Syngonanthus belem and Tonina fluviatilis, but they didn´t form any root. What is the best substrate for this plants? Nutrient rich f.eks. Plant substrat from Tropica or? Should I choose fine or more coarse gravel?
You can't go wrong with any type of substrate. What your aiming for is water chemistry. Since syns don't have a good root system they'll take mostly through their leaves and secondary the roots. Belem has a decent root structure, the rest of the syngonanthus "Not So Much". Easily uprooted and don't grip as much.
Tonina fluviatillis will form aerial roots no bottom roots, these are fine and whispy with what looks like cotton algae attached but is really part of the roots of the plant.