I've found that even though they're very active little guys they're not capable of eating a ton of algae. I'd say easily 8 of them in your tank they're very fun to watch scurry.
However if you're going full out planted tank you're going to need a lot more than 20w of light.
In my 30g tank I have 5 of them! And they are very happy family!
But originally (when I had an algae problem) I put 12 of them…
In less then a week all algae were gun! Then I notes that Otto a start lost the wait (there tummy was not that big like before) I start add supplemental food, and finally move 5 (I lost 2 of them in first week) in another tank.
Now (after almost a year) I have algae free (almost) tank and 5 happy Otto!
