That is a common problem with otos. They get very hungry at the LFS
because there is just not enough algae for them, and you don't know if they feed them specifically or not.
Then, supposedly, even when you get them home and feed them they have lost a crucial enzyme in their gut and can't digest what they eat, and they die.
Pick the otos that have round bellies at the LFS
, so you know they are getting something to eat.
Otherwise, congrats on your new cats!