When I shift-click while in a slabfish's UI and the slabfish has no items equipped, it will delete the items. It does not matter how much of that item is in a stack. It only happens when items are shift-clicked from the middle row of the inventory. I also had a few other mods while playing with environmental. Maybe it has something to do with the compatibility of the mods. The mods I also played with were:

• abnormals core
• abundance
• atmospheric
• berry good
• bayou blues
• better badlands
• endergetic
• just enough items
• optifine
• neapolitan
• upgrade aquatic
• fruitful
• extra boats