[Griznatch]

I'd think that'd be ok, the big deal for them is food. the extras will be ok as long as there's enough biofilm and algae. I feed mine zuchini, dandelion leaves, and algae pellets. They have to compete with the gazillion shrimp I have in there.

 

[Discusluv]

Food access is the qualifier. The tank size is fine for four more.
You say you have had the tank up "for awhile", so the tank biofilm buildup should be adequate. Adding supplemental food would also be wise as was listed in last post.
There really is nothing like watching a large group of Oto's swimming/interacting together in a tank. I totally understand the allure of wanting to just keep them as the sole fish in a tank.

 

[Aquascaper1]

Sounds like you are going to experiment so let us know what you find.

I personally like to have a bit more variety in my planted tank since I think of it as an ecosystem. Otos are cool and I like them in combination with Corys too since each have a part to play in different parts of the tank.

I have a low-tech planted 30 gallon tank with (2) Dwarf Gouramis, (2) Honey Gouramis, (5) Cherry Barbs, (5) Otos, (2) Green Corydoras, (2) Albino Corydoras, (1) Clown Pleco

They all get along, have their own areas they hang out and the Corys clean other areas than the Otos and Pleco.

Just a thought. I am a hobbyist.

 

[ichthyogeek]

Like others have said, food is the issue. I like to feed mine repashy soilent green and supplement it with indian almond leaves. If it's an oto only, I'd say you could probably do 15, if not 20; but this is ONLY if you can keep the tank fed.

 

[carseatdust]

As everyone else has said, feeding rather than bioload likely be the limiting factor here. What are you feeding your current otos? Are they just grazing from the tank, or have you been supplementing with additional food? They'll enjoy munching on that sponge filter, but will need more aufwuchs (algae + microbes) as the weeks go on.
Of the suggestions already listed, I've had success with blanched vegetables (capsicum, cucumber so far) and algae wafers (Hikari). I considered myself lucky, but you might want to have a plan A/B/C. For me plan B that would have been repashy soilent and indian almond leaves as @ichthyogeek mentioned.
Otos eat more or less constantly, like shrimp, so you should be able to look to their behavior to see whether tank is set up to their liking. Maybe get into a rhythm with the 11 you have, and then when you're comfortable that they're getting everything they need you could increase their number?

 

[AdamTill]

If it makes you feel better I kept 30 with no issues in 20 gallons when I was first trying to breed them. Not huge waste producers. Feed lots though. Zucchini, Soilent Green, and red pepper are my favs (mine are breeding right now).