As others have said, a hydrometer makes life much easier. A refractometer is more accurate, but a hydrometer will do just fine for brackish water fish.
I wrote a little computer program called
Brack Calc that will help you estimate how much salt you need by weight. At a pinch, you can use this approach to do water changes until you get a hydrometer. Normal seawater has 35 grammes of sea salt per litre of water. The SG of such water is about 1.024 at 25C. If you wanted one-quarter strength seawater, which would be fine for, say, a figure-8 puffer or some mollies, then you could divide the 35 grammes by 4 to get the amount you need. Brack Calc does that calculation for you by allowing you to simply pull sliders back and forth.
The problem with weighing salt out is that unless you take specific precautions to stop moisture getting in*, it will absorb water. This means that over time, 35 grammes of the stuff is no longer just salt, but also water. Thus, your estimates will become steadily less accurate. This is why you use a hydrometer, because it removes this potential error.
The best approach when raising the salinity in a brackish water tank is simply to decide on what SG you want (say, SG 1.005), make up a bucket of water to that SG, and then add that water to the tank with each water change. Don't worry about the exact salinity in the tank. It doesn't matter. The fish are less bothered by salinity changes that we imagine! They have evolved to adapt to them. So if your tank starts off as freshwater, and you change 20% each week, after about a month or so, it will be as close to 1.005 as makes no odds.
Brackish water fishkeeping is actually very simple, because the fish don't care about steady salinities and don't need a specific SG. For low-end brackish stuff, anything from 1.002 to 1.006 will do fine, and for the high-end stuff, anything from 1.010 to full marine is acceptable. It isn't like keeping a reef tank, where the marine animals expect water chemistry that doesn't change, ever. Even better, you can use changes in salinity to zap external parasites like whitespot.
Cheers,
Neale
* Specifically, store in a dessicator using something like cobalt chloride to remove atmospheric moisture. In the bag, the salt should be water-free ("anhydrous"), but as soon as you open the bag, it will inevitably absorb moisture from the air, even in a dry climate.
Ok I havent got my tank or puffer yet, but how much salt do you add per litre to add to the tank? To increase the SG do you just keep adding more saltwater to the freshwater or just increase the salt?
