There is a plethora of information available about the nitrogen cycle and how it relates to fish keeping, so I’m not going to discuss that here. You should already know that measurable amounts of ammonia and nitrite in your tank are toxic to your fish. You also may be aware that nitrates, while nearly always present in your tank, aren’t as dangerous as long as the measured level is not very high.
“So?” you ask. Nitrate is often largely overlooked. Measurable amounts of nitrates are indicative of a healthy, or soon to be healthy, tank. That’s because its presence suggests your tank and filtration set-up have accumulated enough beneficial bacteria to begin breaking down, or to fully breakdown, ammonia and nitrite (assuming the nitrate origin isn’t the source of your change water). Nitrate is also often overlooked because its toxicity doesn’t match that of ammonia and nitrite at the same concentration (ppm). However, it’s important to understand why nitrate levels should be controlled. If not, it WILL impact your fish. How much depends on the concentration, the exposure duration, and the cichlid species.
Losing fish mysteriously when your ammonia and nitrite levels are zero, and you have no aggression problems? Check your nitrates.
Here’s a nice piece from one of the forum moderators at Aquaponics Nation on how nitrates affect fish, complete with references.