Purely on the fish, I would have said the tank was at the boundary of stocking, on the red side of the line which an over-filtered & experienced keeper may have managed, but as per the other advice, the groups are now right for the fish that you do have. As the tank is on the smaller side too, over-stocking has a much bigger impact. Time to review which 2-3 you really like, re-home the remainder to good homes/local fish store and increase their numbers whilst maintaining a watch on the stock volume.
For the fish you have I would find (and some will disagree) the surface area in inches/12 as a good rule of thumb.
So if you tank is 24"x12" you should not go more than 24" of fish if you were to add their lengths nose to tail. Make sure you can stay within this guide for the types of fish you have but allowing for the recommended minimum shoal to be reached. Also, this is maintaining that fish you keep are under 3" maximum, general about 2" as adult and spread across your aquarium height (e.g. adding 12 otos would just over populate the bottom of your tank.)
begin the wrath....