Need to check the nitrate and nitrite. When cleaning that much water which is ok you need to remember your removing considerable amount which will impact water chemistry. I would check your water at the tap and compare it to your water in the tank. Are you puttin in water remotely close to the same temperature of the tank as well as that can shock your fish as well.
I just went to petco and they tested everything and everything is perfect on both tank. I'm probably going to put one by one in the hospital tank until they get better because they are very stress, oh and I forgot my big tank there's white worms and I told the Petco lady and she told me to get a killer worm, i forgot what's it called, but she told me that it works. That the worms are probably stressing my big fish. But I'm going to put my little one in the hospital tank first because he is very stress he's already on his side because he keeps going crazy in the tank going up and down side to side hitting himself.