Tricky isnt it.
Basically about relative densities.
Boats float because their relative density is less than water. The 300 tonne barge displaces 300 tonnes of fresh water, or 300 cubic metres of fresh water.
Take 50 tonnes of concrete off the boat (assume the concretes dissappeared for a minute) and it now weighs 250 tonnes and displaces 250 cubic metres of water. The water level in the tank will fall.
Now put the concrete blocks back, but into the water, not the boat. The concrete blocks weigh 10 tonnes each, but as their relative density is (approx) 2.5 times greater than water, the volume of water they displace is about 2.5 times less than the equivalent weight of a floating boat will displace.
So although there is the same weight within the tank, there is less volume displacement, so the water level will fall.
The cormorants name was Kevin.