... even a year later ...

[Apistomaster]

Most Heckel Discus from reputable dealers will already be used to eating frozen blood worms(Chironomid larvae) and their exoskeletons and those of Mysids and other crustaceans are rich in calcium. It is a component of the chitononous exoskeletons of these invertebrates.

Heckel Discus are not as delicate as you may be thinking or to put it another way, they are tougher than some think. From the day they are captured through the arduous journey to your tank, that so many live speaks to how resilient they actually are. Heckels are more sensitive to insidious deleterious aquarium conditions over time.

I wouldn't run with pure RO water until you have more experience. If you keep them in water >100 ppm TDS and a pH >7.0 initially you will not have any problems.
I think one should have some experience with maintaining some less expensive species of black water fishes in your first trials working with virtually pure RO water in the pH range of less than 4.5. Once you have experience with these types of aquarium conditions then you can gradually change the conditions in the Heckel aquarium with greater confidence. Heckels can thrive in water with a pH as low as 3.5 which is the high end of some types of vinegar but one needs to have a good understanding of aquarium keeping at the extreme low end of what is natural for them in the wild. The dynamics of a closed systems are very different from large bodies of water in the wild.

[tolga]

Thanks again Larry, my local supplier keeps his heckels in 6.5 ph I believe. I would try to keep to this value at the outset so as not to shock the heckels at the point of introduction. Following your advice, I will gradually lower the quantity of the discus mineral salts I'll be adding to my RO water which would hopefully lead to the reduction of the overall hardness of the tank water untill eventually, the ph would also come down. I have a 150 gallon per day RO unit. I'm planning to fit a large water storage tank inside the broom cupboard, so frequent water changes will not be a problem. Your point about experimenting with cheaper black water fishes makes perfect sense. I could try and raise my level of expertise with some of the small eartheaters I was planning to add to the tank in order to help prevent the fine White sand going anaerobic. Perhaps I can extend the period prior to the introduction of the heckels. Thanks

[Apistomaster]

Tologa,
Your plans outline is pretty much how I make the changes towards a very soft, acid water aquarium.
I would recommend trying Apistogramma nissenji as test subjects as they are found in water as soft as any Heckel and with pH field tests which are 4.0 to 5.0. They are about as sensitive if not more so to less than very high water quality and as a small dwarf Cichlid you can experiement in a 15 to 20 gal tank.
I have advised many through their first wild Discus and many of them had prior experience with keeping and breeding Apistogramma species but were doubting if they were really ready for their wild Discus. My advice to them has been that if you treat your wild Discus as if they are a large Apistogramma spp from the same region a particular species comes from you will rarely go wrong. No Discus are found in the native streams of A. nissenji but many other Apistogramma are found in the Rio Negro and its tributaries like A.pertensis and Dicrossus filamentosus so there are actually quite a few Apistogramma spp and other fish fish you can use as surrogates. Dicrossus filamentosus is a species I have often kept with my Heckels over the years. Like Heckels, D. filamentosus are very beautiful fish but they become even more so as the water become softer and more acidic. But they are very sensitive to any abrupt changes and any deterioration of water quality which makes them good "canaries in your mine".
These are also nice tank mates to keep with Heckels later on when you finally get your Heckels.

[tolga]

Thanks Larry.
That all sounds great. I'll investigate these species you mentioned. This project is beginning to take shape...
All the best
Tolga