Ts
From 0 to 40/50 is a massive load of nitrate in 7 days!
Typically that means overstocked. If it doesn't read zero the following day your schedule is in need of review.
I have 11 clown loach in approx 200G with a Bristlenose and they get bloodworm everyday on top of alternative foods. Most of the shoal is the same size or more than the single oscar.
(4x the water, 10x the fish, half the nitrate)
They don't produce that much nitrate in a week and I mean those weeks where the midweek W/C fails to happen.
J, I'm not sure how you can tell that fish is stunted from that picture? Then I'm no oscar pro! Not very rounded out, that's what I got.
Unsatisfied I went off and found this,
"Now I want to step back here…. remember the quotes from the beginning of the article, where it identifies a 96 hour MDL of 1,341ppm (Wow, that's a lot) for Fathead minnows. That means within 96 hours of being exposed to that level of nitrates the fish died, but it is also 100 to 1,000 times higher than any other fish in this particular study. However, this is the exact same species just mentioned in the above study, where testing identified the physiological effects of nitrates could be detected in concentrations as little as 21ppm (on the exact same species - Fathead minnow) and these impacts became “significant” at 41.6ppm. Think about that. Fathead Minnows are an extremely hardy fish, considered amongst the most hardy of all fish, capable of withstanding levels of ammonia and nitrate far beyond most other fish, and the physiological impacts of nitrates can be detected in this species at concentrations as low as 21ppm. Do we think our Oscars and South American Cichlids can do better? When Oscars and South American Cichlid's have evolved to survive in near nitrate free water and Fathead Minnows are commonly found in polluted waters incapable of supporting other species of fish?"
Taken from,
Is Nitrate Toxic? A Study of Nitrate Toxicity
Nitrate should be as close to zero as possible.