Aloha,
I've seen perfectly awesome tanks with Nitrate as high as 40ppm, but lower than that is better in the long run. The list Bryce gave above is great
but could have even wider ranges and still result in an awesome tank, for example:
Temp 70-80
Salinity 1.022-28
Alkalinity 8-14
Calcium 350-550
Mg 1200-1400
pH 7.9 - 8.4
The parameters interact to form a healthy environment.
It's very very important not to force a well running tank to fit within all the ranges. Too many hobbyists on this forum (do a search
)have ruined their tanks by the addition of chemicals which did no harm at first but which created a dangerous but unseen chemical imbalance that totally crashed their tank in a day.
*** Anyone desiring specific help would be well advised to post a pic of their tank here, stating their current parameters, their specific concerns and asking for comments from the experienced hobbyists that frequent this forum.
Mahalo,
Mark