Bignose - if it is 0.25 ppm and taking the worst case scenarios from your charts - that still 0.02 ppm toxic - which is the threshold in your article. So doesn't 0.25 still stand as a decent guideline?
Would also be pushed to tell the difference between 0 > 0.25 on most test kits anyway?
You didn't read my article close enough. 0.02 ppm is the toxic level of ammonia, not total ammonia. Your test kit measures total ammonia, that is, the sum of ammonia and ammonium. You take your test kit reading, you take your pH and temperature reading, you find the % of ammonia that is in the total ammonia based on the pH and temperature charts I posted and find how much ammonia you have. If that ammonia concentration is above 0.02, then you are in trouble.
That's by just saying a reading of 0.25 ppm on your test is insufficient. If the water is really acidic, it may be just fine because most of that total ammonia will be ammonium. But the water was basic, that could be very deadly. It all depends on your temperature and pH. One single number is not accurate enough.
Let me give an analogous example. Say you are a baseball player and you hit the ball 360 feet. Is that a homerun? There just isn't enough information. Because it matters whether you hit it to center field (usually as deep as 400 feet) or down the lines (usually much shorter than center). It matters if you hit a towering high shot or a line drive because the height of walls are different at different parks. Etc.
Saying 0.25 ppm is the same in that it isn't enough information. The pH and temperature are both critical pieces that have to be added to the puzzle or else it cannot be answered. There are cases where 0.25 ppm is deadly and times when it is almost harmless. Just like there are times when a batter can hit a harmless 360 foot fly ball or it is a homerun. It depends.
In general, no single number is sufficient to describe what really happens in real life. Real life is very complicated, and most things are very non-linear and complex. Sometimes these can be reduced to single numbers, or single rules ("one inch per gallon"), but the reality is that such gross simplification usually grossly inaccurate in a great deal of the cases. This is one of those cases where simplfying to just a single number just doesn't work.
/www.ehow.com/how_2252098_get-lower-...evels-fish.html. Possibly a nitrite/nitrate mix-up? But still frustrating! I've never heard of varying levels of nitrite being idea for various fish. 
