Have I done something wrong?

Got a shot of your tank? Or a list of what plants you have in there?
 
Yeah, I definitely agree specially with a Betta.
 
In a fully planted tank that has little or no nitrates you may need to add some nitrates into the water for the plants to use so you will not be overun with certain kinds of algae.You can try adding some Florish.As far as adding more fish I would hold off on that for now. If this is a new setup I think you need to take a step back and observe for awhile. You do not want to go too fast or you will be setting yourself up for disaster. At the moment, I think there are enough fish in your tank.

Marinemom
 
Well, here's the current stock of my tank:

Code:
Fish:
3 X Xiphophorus maculatus (Orange and red with black spot on tail Platys)
1 X Betta splendens
4 X Otocinclus cocama

Plants:
Sagittaria subulata
Anubias congensis
Ludwigia repens (Ludwigia Broad Leaf Repens)
Is the fact that I have a measurable amount of nitrates evidence that the plants as they are will be fine?
 
If you have some detectable nitrates in the water for the plants, that is good. Adding some Florish will only help at this point. You want to stay one step ahead of the algae game.

Marinemom
 
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