Some fish cannot tolerate any salt. Some breeds of Tilapia can tolerate seawater concentration levels of salt (roughly 35 ppt). Most Tilapia can survive brackish waters if they are slowly acclimated to the salinity (I have read studies of up to 20 ppt for nile and blue Tilapia if I recall correctly with a low mortality rate).

The reason salt helps with nitrite spikes is because the Cl- or Chlorine ions helps offset an imbalance caused by nitrite ions and the diffusion of ions through the gills of fish for their metabolic pathways. Nitrite spikes can cause brown blood disease or something like that... I forget now, but I think I was reading a SRAC article or something late one night...

As far as what rfeiller has mentioned... I know that some fish release growth inhibiting hormones into the water. As the concentration increases (because of the lack of water changes) the fish tend to stop growing and eventually die. In my days researching Tilapia, I have NEVER come across a fact that states Tilapia produce this kind of hormone. Where I have mainly seen this problem cited in studies, it has done with home aquariums and indoor aquaculture with recirculating systems. Why? Probably because a hormone or protein released into the WATER is easily broken down by the UV rays from the sun (or even better a UV clarifier), obviously, since it no longer belongs to a living organism that might aid in its repair or reconstruction... so, with those 2 things said, I doubt anyone, anywhere, would ever need to do "water changes" to prevent stunted fish growth... If you find some studies or references to the contrary, I implore you to post them! Thanks.

P.S. Can you post on the economic feasibility of raising Koi for profit? I am trying to figure out what route to go as far as making a few bucks on the side with my soon-to-be large backyard aquaponic system!