Guppies keep dying??

[Athena99]

Hello,

I am concerned my guppies have tuberculosis. Over the last few months, every once in a while one would develop and ulcer looking bulge near the anal fin and soon die after I quarantined it. The only image I have been able to find online similar to my fish was posted by guppy expert and I have been unable to find any other similar photos. Do you believe that is what my fish has? The ammonia and nitrite léveles in my tank were negligible with maybe 2.5-5ppm nitrate when I tested with the API master kit. I do bi-weekly 25% water changes for my 20 gallon and feed them every 3 or so days. Thank you for help!

 

[Athena99]

Some information I forgot to detail:
1~What type of fish is afflicted? In addition, please describe what is wrong with the fish to the best of your ability (i.e. cotton like growth, bloated, etc.). — guppies
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values. — ammonia 0ppm, nitrite 0ppm, nitrate 2.5-5ppm, pH 7.4, temp 78°F
3~ How large is the tank? How long has the tank been set up? 20 gallon long, set up about 6 months
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of gph if known. AquaClear 50
5~How many fish are in the tank? What kinds of fish are they and what are their current sizes? 7 neon tetras, about 10-15 guppies (mostly babies so hard to count)
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time? Water change and gravel vacc every 1-2 weeks, remove about 25% water
7~How long have you had the fish? If the fish is new, how did you acclimate it/them? I’ve had the fish 4 months, guppies were born in the tank
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.? No
9~What kind of food have you been feeding your fish, have you changed their diet recently? Frozen brine shrimp