Hope this helps to evaluate these important measurements for Ammonia, Nitrite & Nitrate.
Measured 'Ammonia levels' actually measure both ammonium (safe) and ammonia (highly toxic). The pH of the water determines the balance, in this case a pH of 7.6 approxiamtes to 0.44%, or for 0.25ppm, 0.0011ppm of actual toxic ammonia: consider anything > 0.02ppm a lethal amount.
The presence of Nitrite in the water column is particularly nasty since it replaces some of the oxygen in the fish's blood with this poison. Consider levels of below 1.0ppm as 'low', 2.5ppm as 'medium' level, 5.0ppm, although not lethal, prolonged exposure will result in stunted growth & detrimental to a fishes long term health.
Nitrate levels: Ideally below 40ppm. Anything >100ppm could cause problems for fish.
Andy