I don't think that this thread emphasized enough and the freshwater angels are exceptionally intolerant of salt. grimmy, you really need to research these things before you just throw them out there like that. There must be some website that says that angels are brackish, because I know that I've personally has to shoot this down 3 times now.
Anyway, angels come from the Amazon river basin, some of the most soft, acidic mineral-free waters in the planet. The point is, angels are exceptionally intolerant of salt because their bodies never developed the mechanisms to deal with large quantities of salt. Their natural inhabitant never gave them a reason to develop mechanisms to deal with minerals.
The final piece of the puzzle is that angels have become a little more tolerant, because they have been coaxed into breeding in a wide variety of waters today, because they are so popular. That means that they can handle most tap waters, that have calcium and magnesium, but sea salt contains a significant percentage of sodium that angels are not exposed to in their native waters. The concentrations of sodium in the Amazonian basin are measured in part per billion or even parts per trillion, they are that small. Again, their bodies do not have the mechanisms to deal with large concentrations of sodium.