I think the biggest issue with otos is twofold.
1. they are imported in huge numbers and little water. they are often not fed properly upon import. Also, because they are so crowded in the bags, stores assume they are diseased when pale or thin so treat them immediately.
2. Often, importers or stores put them into water treated with antibiotics. Becasue they are an obligate algae eaters with specialized long digestive tracts, this kills all the bacteria in their gut used to digest teh algae that they eat (would otherwise be undigestable). Because of this, they cannot turn the food that they are eating into usable nutrients and end up starving.
If you can get otos that have round bellies and are obviously well, they usually do great. If buying from a store, you might ask how long they have been there, what they are being fed, if they are eating, and if they have been treated wtih antiobiotics.
If they have been treated with antibiotics, dont' buy them.
I get them in in groups of 1000 and rarely have losses.
I feed them a mix of cultured algaes and algae specific gel foods (most recently been trying that of allen repashy which is in prototype) like mazuri.
Supplementation with vegetables can be helpful, but no matter what you offer them, if the bacteria used to aid digestion are not present, they cannot digest the food properly.