50 nitrate max is a limit provided by Mark Vanderwahl and Jake Adams in their reef therapy episode on Algae. Most recommendations are in the 5-15 range. The pollution comparison was made on a BRStv video (likely an episode on nitrate/phosphate/algae control). Seems sensical that if you have high nitrates, the water is polluted as how else would the nitrates get high. But low nitrates doesn’t necessarily mean unpolluted water as there are ways to reduce nitrates chemically. The comment on HLLE is a suspicion (hence the “I suspect”) from personal experience dealing with hole in head on a regal angel.Do not have any idea where you got this from.