a little while ago, i read in a post that TPN was short of an ingrediaent. but i cannot remember what it was. im sure it began with p and the second word may have also began with p. (postassium and/or phospate?)
You're looking at dry salts which are used when following the Estimative Index method of fertilising. Take a look in the Fertiliser section (Not dry powders) for all-in-one mixes such as Tropica Plant Nutrition+
It's difficult to answer. How densely are you stocking plants and fish? TPN is probably okay if you have plenty of fish and a fair amount of plants. TPN+ is better if you have little or no fish and lots of plants.
For dry fertilisers, see the sticky at the top of this section, titled 'EI daily methods or PMDD + PO4'.