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.

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