Fish Not Behaving Normally And Has Damaged Fins

I have a pair of golden wonder killifish. They are still young and haven't grown to the full 4". My problem is that one of them has been sitting at the bottom of the tank and has a damaged tail fin. I moved her to a hanging breeder box for the mean time to try and limit aggression (assuming that the other killi, that I believe to be a male, was being to aggressive with her).

My tank conditions:
15 gallons
Ammonia 0ppm
Nitrites 0ppm
Nitrates 40-80ppm (I can't tell the difference on the API test kits sheet)

Stock
3 Amano Shrimp
2 Golden Wonder Killies
1 Golden Honey Gourami
1 Peppermint Platy
1 Bamboo Shrimp

Much help would be accepted. If it comes down to requiring a bigger tank I already have one it's just needing to be resealed and to have a stand set up and some upgraded equipment.

 

Your nitrates are dangerously high and likely are the culprit.

 

Okay then. All other fish seem to be doing fine. I'll try to lower nitrates but what if that isn't the issue?

 

Then we’ll go from there, I’ll help you. How often do you do water changes?

 

Not often enough. Maybe once a month? I usually just add water when the tank is looking a little low. I know I need to do more water changes though.

 

Once a week is what I’d recommend.