Red fin-like protrusions from gils

Question

[New aquarium owner. Pardon any lack of basic knowledge or incorrect terminology.]

My fish has some weird red protrusions coming out from the bottom part of its gills that look like either "external gills" (if such a thing is possible) or a second pair of fins (again, if such a thing is possible).

I don't really have any basis to go off of, and none of the pictures I found online matched what I'm seeing, so seeking guidance/advice on the matter.

--> Can anyone tell me if this is normal or my fish is sick?

Photo:

Here is another view:

Background:

I "rescued" a few small fish from a local pond that was being emptied for renovations. Most of them died while I was trying to figure out how to care for them and getting the water chemistry right. (There was a lot more to this than I expected...) Anyway, now only one of the fish is still alive. For the purposes of this post let's call him/her "Sam". I want Sam to have a good healthy long life so I can reintroduce them back into the wild after the pond reopens. But I've noticed something that I think may be indicative of a health problem.

Issue and Observations:

I was out of town for a week and when I came back today, I found that Sam had red protrusions coming out from the bottom of their gills almost like a second pair of fins.

• I don't think it's a parasite since it's identical (symmetric) on both sides of Sam's body.
• I see these protrusions "move" when Sam swims around. The motion is very fin-like, but I can't tell if Sam is actively moving them or they're just flapping around due to water currents.
• I don't recall seeing these on Sam before I left, but maybe I just didn't notice because Sam was a grey color before and started turning red during my absence. (Note: The pond where Sam comes from has lots of red fish, so I'm not overly concerned about the color change – yet.)
• Sam does not appear to be lacking oxygen: They're not swimming up to the top of the aquarium and doesn't appear to be "gasping" for air.
• Sam is less active than before I left, but maybe this is explainable by the time of day. (It's night now. I came back late in the evening. So maybe they're just sleepy and less active.)
• Sam's diet changed while I was away: Before I left, I was feeding them Pond Flakes and freeze dried Blood Worms, but in my absence I put in a block of Top Fin: 10-Day Fish Food Feeder

Technical Details:

Fish type:

• Fresh water
• Non-tropical (found in a pond in central Georgia, USA)
• Unknown species
• Just your average run-of-the-mill street fish (you can probably find them chilling in the parking lot of your local mall, or throwing rocks at cars from an overpass) (joke)
• About 3/4 inch long when I got it, and now it's about 1.5 inch long

Aquarium:

• Basic 10 gallon aquarium
• Located in direct sunlight but with with lots of shade and caverns for Sam to hide in. (I tried to replicate the environment of the pond.)
• I have both aquarium sand and colored gravel for the ground cover
• Several small aquatic plants
• Two snails
• A properly sized filter with mechanical, biological, and carbon filtration.
  • Note: I just changed the carbon filter before leaving. So, maybe I didn't wash the carbon enough before putting it in and this caused carbon particulates to enter the water and injure Sam's gills?

Water metrics:

Water chemistry before my absence:

• Temp: 74ºF/22ºC (night) - 80ºF/27ºC (day) (Fluctuation is due to direct sunlight on tank during day) :: OK
• Nitrate: 0 < NO3 < 10 ppm (mg/L) :: GOOD
• Nitrite: 0 < NO2 < 0.5 ppm (mg/L) :: GOOD
• Acidity: 6.5 < pH < 7.0 :: LOW
• Carb Hard: 0 < KH < 40 ppm (mg/L) :: LOW
• Gen Hard: 0 < GH < 30 ppm (mg/L) :: LOW

Water chemistry after coming back:

• Temp: no change :: OK
• Nitrate: no change :: GOOD
• Nitrite: no change :: GOOD
• Acidity: 7.0 < pH < 7.5 :: GOOD
• Carb Hard: 80 < KH < 120 ppm (mg/L) :: GOOD
• Gen Hard: 180 < GH < ??? ppm (mg/L) :: HIGH (The test strip turned purple: This is above 180 ppm, which is the highest my test kit is labeled for.)

Planned corrective measures:

1. I increased the bubbler intensity and frequency to make more aeration. (I had it set on a timer and running at half strength, but I'll leave it on full blast overnight and see if that changes anything.)
2. I've started slowly changing the water in the tank. I plan to do this over a period of a couple days to avoid sudden changes. (A sudden water change is what killed all the other fish I had rescued from the same pond, so even though Sam survived it last time, I don't want to take any risks now; even though the GH is clearly very high now and needs to be corrected right away.)
3. I removed the feeder block in case that's what was causing the pH, KH, and GH to rise.
4. I might play some light music for Sam on my viola: It's 1AM and my neighbors are blasting some awful tunes at their party and probably keeping poor Sam awake... (joke, but no really, can't they go to sleep already??) [update: it's 2AM and they've finally quieted down - these posts take long to write! :) ]

Answer 1

Just to clear up the confusion: the "red protrusions coming out from the bottom of their gills almost like a second pair of fins" is indeed a second pair of fins. They were just translucent enough that you didn't see them and the fact that they are red now indicates a health problem.

I'm not experienced with fish and cannot diagnose this problem, but Trond Hansen noticed in the comments it looks like fin rot to them. The redness surely looks like an infection.

Here's a simplified diagram of the external anatomy of fish by Enchanted Learning. Notice the pectoral fins right behind the gills. They are usually used for steering or slow movement and may lay flat against the body in rest, so they may not be easily visible.