Calculator · Which salt · How much is too much · About specific gravity · References
Goldfish salt calculator
Put in the salinity you want, what the tank is at now, and how much water you have. I’ll tell you how much plain salt to add, in grams, tablespoons, and cups.
Salt is a tool, not a tonic. Don’t keep goldfish in salt all the time, and don’t expect it to cure everything. Work out what’s wrong first (start with Sick fish). Salt doesn’t evaporate and the filter doesn’t take it out. It only leaves when you take water out, so top off with plain dechlorinated water, never salted water.
Calculator
Which salt
Use plain salt with nothing added: aquarium salt, canning and pickling salt, or kosher salt. The label should list salt (sodium chloride) and nothing else, so no iodine and no anti-caking agents. Marine salt mix and Epsom salt are different things and don’t go in this calculator.
The spoon and cup figures assume fine salt at about 18 g per level tablespoon, which is what table, canning, and pickling salt weigh. Kosher salt is flakier and weighs less per spoon: Morton coarse kosher is about 15 g per tablespoon and Diamond Crystal about 9 g. With those, a tablespoon gives you a lot less salt, so use a scale. Grams are the number that matters, whatever salt you use.
How much is too much
- Up to 1 ppt (0.1%): a light dose. Most plants are fine at this level, and it’s where the salt treatments on my medications page start.
- 1 to 3 ppt (0.1–0.3%): the usual in-tank range for easing stress from handling, shipping, and a sick fish’s salt balance.
- 3 to 6 ppt (0.3–0.6%): treatment strength. Hospital tank only, for a set number of days, while you watch the fish. 6 ppt is the most I list for leaving fish in.
- 5 to 10 ppt (0.5–1%): bath strength, for a few hours, not days. In one study, goldfish got through 12-hour baths at 5 and 10 ppt, but 10 ppt threw off their blood salts.
- Around 20 ppt: in the same study, almost half the goldfish became listless or struggled to breathe within an hour.
- About 30 ppt (3%): a dip of seconds to minutes in a bucket, never a tank dose. See salt dip.
Go up in steps. Dissolve the salt in a bucket of tank water and add it in parts, not all at once. Start at the low end and only go higher if the low dose isn’t doing the job.
Goldfish handle salt well, but plenty of other things don’t. Some plants die above about 2 ppt. Snails, scaleless fish, some catfish and tetras, and fish that sense electric fields do badly in salt. If your tank has any of those, treat the goldfish in a bare hospital tank.
To bring salt down, change water. A 50% change with plain water cuts the salt about in half.
About specific gravity
If you pick SG, I convert it with the standard seawater equation, for a reading taken at 77°F against pure water at 77°F. At these low levels it works out to about 1 ppt for every 0.00076 of SG, so 1 ppt is about 1.0008, 3 ppt about 1.0023, and 6 ppt about 1.0045. A swing-arm hydrometer can’t read that fine. A refractometer or salinity meter that reads ppt is better, and weighing the salt is better still. Meters and refractometers are set up for seawater, so with plain salt their readings are close but not exact.