08-09-2008, 03:38 PM
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I think salmon are a little different, since they are born in entirely fresh water far inland, then move out to the open ocean later in life. I could be wrong, but I think puffers tend to spend their whole lives in what could be called "brackish" water, but move to saltier and saltier areas as they grow.
You can use a hydrometer to measure the salinity of your water. Refractometers are more accurate but also more expensive.
For a green spotted puffer, you would want to match the salinity of your tank to the tank at the store you're buying your fish from. Otherwise, the fish could suffer osmotic shock and die. Then, every other water change or so, you'd want to add a little bit more salt to your new water than the previous measure to slowly build up the salinity of your water to the desired amount for a full-grown fish. It sounds complicated but it's actually not too tough, since the salt doesn't ever disappear from your aquarium except by water changes. For example, as long as you keep your tank topped off with additions of fresh water to make up for evaporation (where no salt is lost), you could remove a gallon of water with a specific gravity of 1.005 and replace it with a gallon with a specific gravity of 1.006. This would very slowly and safely increase the salt level of your tank. (Of course, I haven't exactly worked out the math here, so depending on stuff like the size of your tank and how fast your fish is growing, you would add salt more slowly or faster than this.)