Because the water height of the three fish tanks is the same, but their volumes are different, their bottom areas are 20, 24 and 30 square decimeters respectively.

The total volume is 20*2+24*2+30*2= 148 cubic decimeter.

1. Pour the water from the tank with the largest bottom area into the tank with the smallest bottom area, and the water surface is the highest.

30*2/20+2=5 decimeters

2. At this time, the volume is 20*5= 100 cubic decimeter.

3, 148 liter, which has been calculated above.

4. Pour the water from the tank with the smallest bottom area into the tank with the largest bottom area, and the water surface is the lowest.

20*2/30+2=3 and 1/3 decimeter.