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romaine lettuce is good. All you have to do is cut of the non-green or core and freeze it. This breaks down the cell structure and prevents "blockages". Also helps to remove air pockets in the lettuce.
There are several different varieties of dried seaweed that your hippo would enjoy. Just remember to remove the uneaten seaweed after a few hours so it doesn't foul the tank.
Uneaten? After a few hours? I didn't know that was even possible. Mine never last that long though with the pigs they are there's pieces floating everywhere