I was able to exchange some emails with Dr. Neale Monks this past week, discussing nitrates as I had promised, so I will add some clarification and new information received from Neale. My original post, #7 in this thread, pretty much stands as it is; it was based upon research including articles by Dr. Monks, so that is not surprising, but I may be able to make some of it a bit clearer.
As to the unit, when Neale refers to keeping nitrates below 20 ppm he is doing so with the basic hobby kits like the API in mind. He suggests that more emphasis be placed on the colours than the numbers with these kits. Referring specifically to the API, he says its card ranges from yellow (low) to red-brown (high). Nitrates in the yellow or orange are fine; in the red is not good for sensitive fish (he specifically mentions dwarf cichlids and mollies), while red-brown is not good for anything long-term. The aim should be to keep nitrates as low as possible, and live plants and water changes are the best ways to achieve this if nitrate is an issue as with sensitive species. He notes that excessive nitrates are usually resultant from poor maintenance, overstocking, or overfeeding.
Just to put some numbers to these colours for those without the API card...yellow is zero, yellow-orange is 5 ppm, and orange is 10 ppm; orange-red is 20 ppm, red is 40 ppm and 80 ppm (I cannot tell the difference), and brown is 160 ppm.
I asked Neale about issues caused by nitrates, and he confirmed that over the long-term, high nitrates will stress fish, causing a weakening of the immune system creating an opportunity for disease, and a shorter lifespan. He mentioned that hexamita and hole in the head in cichlids are now associated with high nitrate levels, and he agreed that Malawi Bloat may well be due to nitrates, and keeping nitrates below 20 ppm for all cichlids is advisable.
We briefly discussed the studies on commercial fish and how they may relate to the hobbyist. Neale pointed out that nitrate is complicated. The lethal level of nitrate for some fish experimented on in labs seems to be high, supposedly 100 ppm or more. On the other hand, scientists have only tested a very few (big) species, farmed trout for example, and often their experiments are shorter term things, like how many of the fish die across, say, a week or a month. We're keeping a hundred different species often for years if not decades, so our experiences (and expectations) are different.
Hope this helps.
Byron.
