What is this Orange Lump on My Fish?

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mawgui

Aquarium Advice Newbie
Joined
Jan 12, 2025
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4
Location
Detroit
I noticed this orange swelling/sore on my fish about four days ago. I immediately did a water change and replaced my filter (which had just died). I noticed the lump on his tail today - do not know how long it has been there. I prophylactically dosed the water with KanaPlex.
This very pretty 8" fish was purchased a number of years ago, to keep a (now deceased) fish company, from the feeder fish tank at Petco! I have not noticed any changes in behavior.
What is this and how should I treat it? Thanks!
 

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Red/ bloody appearance internally is usually bacterial Septicemia but could be viral. This is usually brought on by dirty water or the fish being under stress. You would need to use a medicine for internal infections. I would need to know your water's pH and Hardness level to recommend the proper medicine.
The area you have circled in pic#3 can be either a benign tumor, common in goldfish fins, or a bacterial accumulation. The issue is one is treatable and the other isn't. :^s So let's go through these questions:
What size tank is the fish in? Are there other fish in the tank with this one?
What are the full water parameters?( pH, ammonia, nitrite, nitrate and general hardness) Are you using test strips or liquid tests?
What is your regular maintenance routine?
Was there any change to the fish's diet right before the redness showed up?
How long ago did you treat with Kanaplex and did you do a full course of the treatment? ( A single shot of Kanaplex will not usually solve a problem. )

Let's start there. (y)
 
Thank you for your quick reply, Andy! My fish is alone in a 20 gallon tank. I'm using an Imagitarium In-Fil 20 filter since my Aqueon AT-40 in-tank filter [disappointingly prematurely] packed it in about a week ago. (That heater, by the way, is not plugged in :)

I just perfomed liquid tests and found nitrite is 0 pm, nitrate 0 pm, pH 6.0, ammonia 0.25 ppm. There has been no change to diet or maintenance for probably four years (except that I have become more diligent in the past 8 months or so, since the other goldfish passed away). My maintenance routine is to change out about half the water every 7 - 9 days. I administered KanaPlex 4 days ago and again last night. Will do water change again now because of ammonia reading.
 
The ammonia
Thank you for your quick reply, Andy! My fish is alone in a 20 gallon tank. I'm using an Imagitarium In-Fil 20 filter since my Aqueon AT-40 in-tank filter [disappointingly prematurely] packed it in about a week ago. (That heater, by the way, is not plugged in :)

I just perfomed liquid tests and found nitrite is 0 pm, nitrate 0 pm, pH 6.0, ammonia 0.25 ppm. There has been no change to diet or maintenance for probably four years (except that I have become more diligent in the past 8 months or so, since the other goldfish passed away). My maintenance routine is to change out about half the water every 7 - 9 days. I administered KanaPlex 4 days ago and again last night. Will do water change again now because of ammonia reading.
The ammonia reading is actually not an issue. Ammonia in water with a pH under 7.0 naturally converts to ammonium which is not toxic to the fish unless in extremely high levels.
Has the fish grown up in that 20 gal tank? I ask because that is a very small tank for comet goldfish. Since goldfish are very dirty fish, the water should be changed more than once a week-9 days. Here is the new issue tho, if your replacement water for the water change is higher in pH than 7.0, you need to change less water more frequently so that you don't change the pH too much or too rapidly.
Now for the problem, ;) Kanamycin is more effective in higher pH water so at 6.0, it's basically useless. You should be using Nitrofuracin or minocycline. Minocycline is the main ingredient in Maracyn 2. Nitrofuracin/ nitrofurazone is the main ingredient in Furan-2 or found in powder form by itself.
I would do a 50% water change ( do less if your replacement water has a higher pH than 6.4-6.5) to dilute the kanaplex then run some carbon in the filter overnight to remove what remained. After that, use either the Maracyn 2 or Nitrofurazone and follow the directions for water change schedule based on which med you use. (y)
 
Thanks, Andy. Yes, the fish grew up in that tank, along with a couple others (RIP). He was once about 1-1/2 inches long. My middle school daughter picked him from among the countless feeder fish (two for a dollar or less) because she felt sorry for the only silver guy! Now she's moved away and I'm the only one who cares, however inexpertly, about his future.
I just did a 50% water change. I'll order some Naracyn 2 and/or Nitrofurazone from Amazon or Chewy and move forward from there. And I'll monitor water pH.
Thanks again! Anything else I should know?
 
Thanks, Andy. Yes, the fish grew up in that tank, along with a couple others (RIP). He was once about 1-1/2 inches long. My middle school daughter picked him from among the countless feeder fish (two for a dollar or less) because she felt sorry for the only silver guy! Now she's moved away and I'm the only one who cares, however inexpertly, about his future.
I just did a 50% water change. I'll order some Naracyn 2 and/or Nitrofurazone from Amazon or Chewy and move forward from there. And I'll monitor water pH.
Thanks again! Anything else I should know?
Make sure your replacement water's parameters are not far off from the tank water. A rapid change in pH can adversely effect fish.
I will add that even tho the tank is where the fish grew up in, it really has outgrown the tank and you are seeing the effects of this. If you can get it healed, I would seriously consider either a much larger tank or rehoming the fish to somewhere with a larger tank or pond and get fish more appropriate for a 20 gallon tank. (y)
 
Andy, An update -+ I treated my goldfish with Koi Nitro Nitrofurazone powder dissolved in 20% water changes every 2 days for 4 applications. By the third application both the fin and the lump in the tail were healed!
I have confirmed your concern regarding the water changes: the "natural" pH of the tank is 6.0 but the tap water is 7.0 si 40-50% water changes were spiking pH to about 6.4 (measured with test kit). I have implemented your suggestion of smaller but more frequent water changes.
I am watching Craigslist for a bigger tank. Unfortunately the older gentleman up the street with the outdoor goldfish/koi pond has passed away. I know he/she will be happier with company.
Thank you again for your sage advice!
 

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