I would like to know what the formula of salt per 100 gallons of water is. I have some stressed fish...

Here you go--

Formula for Salt in Koi Ponds:

1.0 lb salt/100 gal = 1.25 ppt (parts per thousand)
1.5 lb/100 gal = 1.88 ppt
2.0 lb/100 gal = 2.5 ppt
2.5 lb/100 gal = 3.125 ppt

For amount of salt to add to get to a desired concentration in ppt:

Lb salt to add = (total gallonage of your system/120) multiplied by (desired salt conc. in ppt - current salt concentration in ppt)

For example:

Pond system volume: 4400 gal
Desired salt concentration: 1.5 lbs/100 gal = 1.88 ppt
Current salt concentration = 0.6 ppt

Lbs salt needed = (4400/120) x (1.88 - 0.6)

= 36.666 x 1.28 = 46.99 lbs salt

Remember to add gradually, and preferably not into your filter inlet feed. Waterfall or stream is best.

Please note that salt as a constant additive probably confers no provable benefit to the health of a fresh-water fish. It is useful in early spring to prevent methemoglobinemia ("brown blood disease") caused by accumulation of nitrite as the populations of bioconverting bacteria emerge at their own characteristic rates from dormancy. 1.5 lbs per 100 gallons (1.88 ppm) is probably enough. Wash the levels back down towards zero with water changes as your water warms up and your bioconversion fully kicks in.

Note that the above formula can be manipulated to give you a fairly accurate figure for pond volume if you know how much salt you added and have salt concentration figures for before and after the addition.

Bob Passovoy
President
MPKS