Good point. My grow tanks are set up outside my house. I siphoned water down to 3 inches or so and at the same time hosed in fresh well water for 15 minutes. I then filled tank back up. Did this twice. So more like 90% change. So drastic turn over.
Guppies didn't really show too much distress but sams and otos where in trouble.
Person I got the fert from mixes dry blends of fert with water. Will ask later what it could be.
It's quite possible that you just put in too much at once. Tell me you didn't just dump a bunch of dry fert in to settle as a cloud.
Wait a minute here. I am not a serial fish killer. I do not get thrills watching fish labor while I pour 2 lbs. of granular palm special into the tank. The product I used was home made yes but carefuly meausered and put into 1 gallon plastic containers in liquid form and then I dumped it into the tank.
To prove or disprove the 100% theory one would need to tint the water with color and then see how long it would take with flow through system to clear water.
Fish are 100% better by the way. I just poured in way too much fertilizer formula. I was lucky.