When ModernFix and Lithium are both loaded, the player will receive falling damage when jumping into the water.
The amount of damage is exactly the same as falling directly to the ground.
Removing anyone(ModernFix or Lithium) can fix this.
And those fish somehow died:

ModernFix version: modernfix-mc1.18.2-fabric-2.4.1-62-gb744cc7(this)
Lithium version: lithium-fabric-mc1.18.2-0.10.3

latest.log
I get this log when both ModernFix and Lithium are loaded.

Also, there's another issue that I encountered before:
Days ago, another player connected to my local server through local internet. I have ModernFix installed but he didn't.
When playing, I found that he cannot swim in the water. More specifically, there is no resistance and floatage in the water, for that player. Jumping in the water was exactly the same as jumping on the ground, for that player. But the behaviour of water is completely normal on my side.
Removing anyone(ModernFix or Lithium) can fix this.
I was using an older version of ModernFix back then, I dont know if it's fixed now.

Thanks, should be fixed in the newest build.