I've written this elsewhere before, it summarises what I think.... I have to say gold nuggets are maybe my favourite freshwater fish so good luck to you and do it right!
As a note the Xingu is not a blackwater river, but a vigorous 'whitewater' which means it has almost no hardnesss but a neutral pH, however they are nit fussy. Also a chap local to me has bred these boys, and to get there was using mucho water flow and apparently muchowater changes. If I was to try I'd stick em in the basement so I could fill it from a tap and overflow to the floor drain
Just some notes re: the lady who lost her gold nugget and clown Plec... First of all you're right to ask how much food made it to these plecs - I believe that the majority of these fish (like many L -numbers) die of starvation due to both inadequate and incorrect diet. Notice how so many people complain they grow so slowly, well there's a good reason. I have a few baby Bristlenoses I kept in a tank and hammered with food, they're about 5 months old now and the largest is nearly 3 inches. I remember keeping a bristle in with some Mbuna, and that [fish] just stopped growing when it went in there, in contrast to its brother who is now a good, fat, fully grown 4 inches. Also, as you point out the fish, esp. the gold nugget do not feed on algae, except in utter desperation, rather they pick up worms, bugs, critters and chew on the biofilm I believe. These fish are fussy on water quality and also water movement and dissolved oxygen content. Notice how this lady's and so many hung on the filter outlet. In the only good breeding report I've seen a powerhead was pointing at the spawning cave! 30% a month is thoroughly inadequate, and that LFS was pretty stupid to say so. My fish respond well to 10, 15
percent 3 times a week. I only gravel hoover every 2 or 3 weeks though to maintain a biofilm of mulm for continual chewing. I don't think these fish are too fussy about pH within reasonable limits, but I'm pretty sure nuggets at least are from acidic blackwater rivers (Lower xingu, but I need to check that). There is a theory that these low pH rivers are not terribly bacteria friendly, so fish from these areas are all prone to bacterial infection as they simply don' have a 'bacteria unfriendly' immune system - (examples wild caught Apistos, discus, L nos). Again, another reason for those frequent water changes. So my bet here is a slight slip in water quality, plus a minimal diet caused a bacterial infection and pop. You might not even need the
bacterial bit to kill a slowly weakening fish. So I would say if you're going to keep these fish be prepared to overfeed the tank and counter it with frequent small water changes.