Just how new are the otos?
First thing to do is check water parameters (ammonia, nitrite, nitrate, and even pH, GH, KH, temperature as they can indeed be really helpful figures especially on aged tanks)
Get those test results before doing a water change (you can fill the test tube vials with tank water, then do an immediate water change before testing those vials if you feel it is an emergency to change the water).
Just would like to know some other info. How often would you do water changes and what % of water volume change?
Is the tank dirty, debris-wise?
Really double check all the fish and look for any symptoms that may help ID what is wrong (physical signs and behavioral). Even any small signs, such as red gills, faded color, inactive, rapid breathing, gasping at water surface, flashing, darting, swimming odd, fin rot, whitening of eyes, increased aggression, more skittish, some fish no longer hanging around their normal group, fish skinny, white/clear poop strings, bloated, red blood streaks in fins or body, etc.)
On a side note, maybe we can ID your tetra species. Pic would help most, but a description may suffice.