A guy named Attila Lukács from Budapest bred Rio Negro Heckel eairlier this year.
Below is a quote from a post about the water perameters.
"Here are the 'happy water parameters' as of yesterday evening:
pH: 4.7, EC: 60mS, NO3: somewhere between 0 and 10.
The pure RO-water used for the w/c's has an EC of about 10mS, the extra 50 comes from the life in the tank (food, etc.). The tap water I mentioned earlier is no longer present (or detectable) in the tank, it was only added before the long vacation to provide a bit of pH buffer while a not-so-very-competent person looked after the fish.
Otherwise there is nothing added to bring the pH down, time and the filtration takes care of this. Attila used to filter the prepp'ed water through peat, but he gave up on it as the fish didn't like it nearly as much as the pure RO. (I had the same observation re Xingús and peat.)
The parent fish arrived in early March 2009 as semi-adult fish, since then they have been in the same tank that's located in a corner of the kitchen, right next to a door, so there has always been relatively heavy traffic around the tank, which they seem to have gotten used to. They are fed primarily on a variety of frozen foods, discus granules and occasionally on fruits. What they accept and enjoy of the latter is very much mood (or season, flavour, etc.?) dependant. (Again, the same as with my Xingús.)
Approx. 36 hours after spawning, more than 90% percent of the eggs look nice and viable, the odd fungussed ones the parents pick out. There are about 100 eggs as we speak. More pics to follow tomorrow. Temp was 29C."