It could be they dont survive the drowning-it might just be part of the life and death cycle. The seeds may need to be submerged to sprout-or pass through a fish gut.
I have some of those Amazonian vegetarian fish and I feed them greenery like lettuce and peelings of squash or cucumber. They love corn kernals. When I had the huge 20 pound Pacu fish-they enjoyed Banana and whole grapes-seeds and all.

Hi
To my knowledge there are no broms or epiphytes in that catagory. Could'nt identify the species of brom in the pic but looked quite ordinary. If they all die how does the seed start over?? Also the tree holding the brom was at least 20 feet tall. It grew that much in one season!!!
This happens every year and takes at least 3 months to recede. Fish swimming in the treetops is interesting for sure but epiphytes surviving this treatment is mind boggling lol I certainly wouldn't keep any of my broms underwater Thanks gary