What about tanks with high par? Where do you feel No3 should be if you wish to keep clams?The fluted giant clam (Tridacna squamosa) increases nitrate absorption ... https://link.springer.com/article/10.1007/s00338-020-01907-9
I believe that higher nitrates are a key in aquariums where par levels are lower. Just my somewhat informed opinion combined with personal experience.