I use it after every water change and have never had any fish lose afterwards so I guess it's working.
Love the fact that its dry and will be stable for a while. If you have anything bigger than a 20 gallon get this.
Tough to dose for small water changes. 1 tsp for 40 gallons = 0.05 tsp for 2 gallons. No clue how to measure that little
Just mix this in with your fills and water exchanges. It last longer then your liquids and does a superior job.
Easy to use and very cost effective.