One of my lemon tetras and one of my tetra serpae have a black stain (not only stain, but actually has some texture to it) just before the tail fin. Both fish are swimming, eating, behaving normally. The tetra serpae also got some brownish stain and texture just before the dorsal fin.
What could this be? See attached pic.
I will try to answer as much of the questions from the stick post as I can. Thanks for your help.
1. What is the size of your tank? 40 gal.
2. What are your water parameters? State the brand of test kit used. Ammonia, Nitrite Zero. Nitrate low. pH: 7.0. Hagen Nutrafin test kits.
3. Is your aquarium set up freshwater or brackish water? freshwater
4. How long the aquarium has been set up? 2 months, but moved 2 very "dirty" filters from already established tank and have been carefully monitoring parameters and added Prime at the beginning when/if ammonia traces showed.
5. What fish do you have? How many are in your tank? How big are they? How long have you had them? Lots of Tetras. 20 very small cardinals, 7 lemon, 7 serpae, 8 black skirt, 3 hatchet, 6 silver tetra, 7 harlequins resboras and 1 adonis pleco. ah..... an 1 juvenile angel fish (I'm aware of the risks). The fish have been with me for at least 9 months, except for the cardinals, 2 months.
6. Were the fish placed under quarantine period (minus the first batch from the point wherein the tank is ready to accommodate the inhabitants)? Most of them no. The cardinals, yes.
7. What temperature is the tank water currently? 29-30 C. I'm in Panama, very hot.
8. Are there live plants in the aquarium? Yes. Plenty. This is an El Natural (soil substrate) aquarium.
9. What filter are you using? State brand, maintenance routine and power capacity. Hagen Aquaclear 50 and smaller Tetra. Both HOBs.
10. Any other equipment used (aside from heater and filter which are two very important components of the tank)? No.
11. Does your aquarium receive natural sunlight at any given part of the day? What is your lighting schedule (assuming you do not rely on sunlight for our viewing pleasure)? Little sunlight (indirect from window). 80 W of fluorescent light for 12 hours per day.
12. When did you perform your last water change and how much water was changed? How often do you change your water? Do you vacuum the substrate? I changed 25% a couple of days ago, but the fish have had these stains for some time now (months?).
13. What foods do you provide your fish? What is the feeding schedule? Tetra tropical flakes twice a day.
14. What unusual signs have you observed in your fish? Just the textured stains.
15. Have you treated your fish ahead of diagnosis? If so, what treatments did you use? State your reasons for planning ahead of proper diagnosis. Just a little Melafix the other day (half recommended dose). Did not notice any change.