Hi all,
Chris is right you can't make really saturated stock solutions as all the salts in solution are additive, and sooner or later the one with the lowest solubility will come out of solution (re-crystallize). Salts are also more soluble at higher temperature, so as the temperature of the stock solution cools, salts will come out of solution and may be difficult re-dissolve even if you re-heat the solution.
Some salts are very soluble (potassium nitrate - 360g/1000 mL at 25°C), but many phosphorus compounds for example are much less soluble, so you might add the very soluble KH2PO4 (monopotassium phosphate), but it could re-combine with the calcium (or magnesium) all ready in the solution and precipitate out as tricalcium orthophosphate Ca3(PO4)2 or similar.
As long as the salts are in solution and the bottle air tight concentrated stock solutions keep for long periods, and because they are concentrated you don't get the problem of algae growth (weak "Long Ashton" or Hoaglands solutions must be kept in the dark for this reason.) The HCl is to acidify the solution and keep nutrients in solution, rather than as a "preservative".
A further problem is that Iron chelate FeEDTA is photodegraded by light so must also be kept in the dark, and the non-chelated iron forms insoluble ferric phosphate (FePO4) very easily.
As suggested the best option would be to keep 2 separate solutions -
A PMDD+PO4 formula for macronutrients.
A micronutrient traces solution (kept in the dark).
A final point is that the "Fluidsensor" Iron + traces mix James Suggests for his "all in one solution" <
http://www.theplantedtank.co.uk/allinone.htm> is very reasonably priced at £5.50 for 100g <
http://www.fluidsensoronline.com/ep.../Products/PN-100-TR/SubProducts/PN-100-TR-100>), and a 100g is going to last a long time.
cheers Darrel