RTR wrote:Brackish water or marine water should be prepared in advance in a separate container and checked for proper specific gravity/salinity. Fish may be adapted to specific gravity changes within hours by drop-wise acclimatization. Unfortunately, the required bacteria cannot, but must be adapted by much slower changes of no more than +/1.002 per 48 hours minimum or slower during the critical range from ~1.008-1.012+ or slightly more. Rapid changes in specific gravity may kill the require bacteria quickly, leaving you with a toxic tank from lack of the beneficial bacteria plus pollution from the death of the bacteria needed for stability.
While fish in the wild may move of their own volition through water of quite different salinities, bacteria and many invert cannot. Required bacteria and other microscopic life necessary for tanks are much less adaptable. To manatain a cycled and well-balamanced tank, you must hand the system suitably for all of the required living components. Statements to the contrary as given above are false.
HTH
Granted between the 'critical' range where bacteria does not adapt as well then you 'may' experience difficulty. I'm not sure what and how much microscopic life is in the tank, I'm also not sure where the majority of the microscopic life exists; be it the water column or the substrate - probably both, I would have thought that much of the microscopic life in the substrate would be little affected as they would be protected by the sand/gravel layer. What is the life expectancy of the microscopic life in the tank - days, weeks, years? presumably they are reproducing and dying all of the time but we don't carry out water changes daily to compensate.
Many people on this forum (myself not included) add a dehclorinator directly into their tank before they add tap water, obviously it would take a while for the dechlorinator to mix with the tap water but peole do not report ill effects from chlorine or heavy metal poisoning causing a toxic tank. I would have thought that this practice would be on a par or potentially even more destructive than what the OP is proposing.
It appears that from Pufferpunk's experience that adding the salty solution in one go will not produce a toxic tank.
Maybe the OP could 'give it a go' as well and report their findings?