Osmotic shock is related to a difference in mineral content, the hardness and TDS of water. Do you know the gH/TDS of the water that you removed and the water you used to replace it? Yes there was a difference in pH but IME, that isn't as important as the TDS of the water.
Any time fish start gasping it is serious. They may live but the stress leaves them more suseptible to other problems.
Yes maybe doing big water changes when the pH is closer would be better. But I would still expect the fish would be stressed if the hardness, salinity and TDS are significantly different. I would look at that first.
Don't know what kind of fish you have. some can handle changes in the mineral content of water better than other. Fish shouldn't look worse after a WC. If mine don't look thrilled by the fresh water I start looking for what is wrong.
My gH is normally 16-18 I believe and since I introduced ADA aquasoil amazonia II last week, all hardness parameters dropped dramatically as suspected.
when usually 7.0,
kH used to be 18 now only 3
gH is about 12
now...before it was 18 or more
I am not sure about total dissolved solids...I cleaned my filters a few weeks ago. Too much silicates..
The fishes I have...
Bosemani rainbowfish, congo tetras, rummynose tetras, cardinal tetras, red line torpedo barbs, golden white clouds, and sterbai corydoras.
My rainbows seemed to be unhappy about it, just gasping. Tetras went along fine. White clouds were worse, they were gasping and gasping at the surface. Red line torp barb was unhappyingly gasping as well.
Thank goodness all of them are alive right now.
Did you treat the tank with Prime or similar dechlorinator? I have never seen fish be unhappy with water changes, even when I have changed twice in an hour at 75% of the water. The fish normally appear delighted with the new water.
Yep, I use prime all the time.