There's a very good reason I routinely write that Rift Valley cichlids be kept at nitrate levels as low as possible, and preferably below 20 mg/l -- the advice I give has to be as close to 100% reliable as possible. I make a living from writing for fishkeeping publishers, and that means if I say something that's not true, my reputation diminishes. Any advice I give has to be as trustworthy as possible. When I make mistakes, or serious experts tell me I'm wrong, I have to make sure that I change my advice in future.
Of course, I don't doubt some people maintain hardy Malawian cichlids at 50 mg/l or higher. But that won't work for all Malawian cichlids, and it'll cause real problems for Tanganyikans. If you understand that nitrate is particularly toxic to cichlids, and you work to lower nitrate levels as far as practical, then you are maximising your chances of successfully keeping the widest range of cichlids.
The analogy with people being scared about travelling faster than a horse is misleading. Nitrate toxicity to cichlids isn't a myth or illusion, and in fact the negative effects of nitrate on fish generally have been widely studied under laboratory conditions. I suggest you spend a little time reading the science; in some cases, concentrations below 5 mg/l can have demonstrable effects on things like adult fertility and the growth of fry. So the idea nitrate is "harmless" as stated in many aquarium books is highly misleading.
Nitrate is also a good proxy for water quality, specifically, water changes. People who maintain aquaria with low nitrate levels are likely doing regular water changes. As a consequence, their fish are more likely to remain healthy. There appears to be connection between infrequent water changes and the occurrence of
Hexamita infections and hole-in-the-head disease. Nine times out of ten, when you see cichlids with
Hexamita, they're in an aquarium that hasn't had regular water changes.
Cheers, Neale
I can't see the value in telling someone they need close to zero nitate, when 20ppm is obviously quite acceptable. Hell 150 years ago we were all going to die from asphyxiation if we travelled at more than 5 mph on a train.