That seems about right. But there's no point dumping all the salt in at once. You need to leave the bacteria in the filter some time to adjust. So each week, do your water changes, say 25% each week, and replace old water with new water at the higher salinity. Let's say you change 25%, or about 9 litres.
9 litres at 9 grammes per litre = 81 grams
81 grammes / 6 = 13.5 teaspoons
13.5 teaspoons / 3 = 4.5 tablespoons
So I'd add 4.5 tablespoons of salt to every 9 litre bucket of water. Stir in and leave to settle for 20 minutes if you can, and then add to the tank. By doing water changes like this over several weeks, you won't upset the filter bacteria.
All this said, most aquarists use a hydrometer for measuring salinity. It's a lot easier. A cheap and cheerful glass hydrometer should cost you around $5-10. All you do is mix the salt and then dump the hydrometer into the bucket and check you have the right salinity. It actually doesn't need to be terribly accurate because your pufferfish couldn't care less. There's a bunch of brackish water articles over in that subforum, so you might care to read those.
By the way, I have a little computer program that helps you instantly calculate how much salt you need. It's called
Brack Calc, is free, and runs on Macs and Windows and Linux. It also switches between Metric and US units, if you prefer to work with gallons and ounces.
Cheers, Neale
