Oto With Red Gills/chin

[Katty]

I noticed tonight that one of my otos has bright red, symmetrical lines on its chin(gills?). It's really scary looking, and paired with the fact that one of my cories is mysteriously ill (link, feel free to add your 2 cents on that too if you want...) it has me on alert. I got these otos I think around a month ago. The other oto has none of these marks. The otos are doing nothing strange, just stuck to the glass.

I just tested the ammonia, which is 0ppm. I tested the nitrite on friday and that was 0, and did a 20% waterchange on Saturday. My nitrate test doesn't work. There's a bit of mulm on the substrate that I'll vacuum up tomorrow. The tank is 20 gallons and planted. Tank mates are bronze/albino cories, keyhole cichlid, green neons and an olive nerite snail. Other tank-mates are healthy with the exception of the one albino cory who is see-through at the moment. I dose 2 mL flourish excel and 1mL iron plant supplement daily.

Pictures:
Bottom
Side

I saw a couple things on google about otos with red gills but I couldn't really figure out a cause other than ammonia. Crossing my fingers that this is nothing, but what do you all think it is?

 

[loraxchick]

Had the fish been zooming around the tank and you noticed this afterward? Happens with otos when they are frantically cleaning/zipping around and then rest. Its just the blood vessels around its head are dilated making the blood-flow easy to see. Id bet that after 10-20 mins of relaxing it goes away. Ive noticed similar "redness" with cories as well so i wouldnt worry too much. Just keep an eye on them and do a big waterchange if you are unsure. Doesnt really look sinister to me-just like hes been busy being an oto.
cheers

 

[Katty]

Hmm, hopefully that's it. I checked again today and it still has the red, only one of the otos. They're both munching on algae like little pigs right now.