Which Fish Should I Get A Buddy For?

[Fc1700]

Right now I have a 10 gallon with
 
Serpae Tetra
Dwarf swordtail
Female Betta
Fancy Guppy
Otto
 
Which fish do you think would benefit most from having another one of the same species? Keep in mind I would get another Otto but I'm nervous about the amount of food because I can't get him to eat algae pellets because my other fish eat them so I'm not sure if i put two, I could accidentally starve them (not enough algae).
 
Thanks for any advice.

 

[lrhodes]

Your tetra. But I would make sure all those fish are OK in a ten gallon.

 

[Ninjouzata]

Serpae tetra. They should be kept in groups of 6+ since they're schooling fish. The footprint that Seriouslyfish gives for them I think is bigger than a 10g though.
Same for the otos but can see why you wouldn't want to add more.
 
The betta is fine on her own.
 
*Edit* Forgot to add, swordtail should be in a bigger tank as well.

 

[mrstwalker]

tetra for sure although your swordtail and guppy would probably like some more friends too