brackish water is roughly around 1.010. 7cups of marine salt should have brought the sg over the 1.012mark. Thats why when you mix marine salt you should wait 24hours before adding it to the tank. I used three and a half cups for 25 gallons,and the sg read 1.012. I use a 32gallon Brute garbage can on wheels,with a magdrive5 to mix it.I have a Stealth heater in the can also.