Red Area Behind Head On Black Skirted Tetra

I added 2 new black skirted tetra to my 20 gallon tank maybe 3 days ago - they joined 3 existing ones from several days before that, one dwarf flame gourami, and 3 guppies. All fish are male.

I noticed tonight that one of the tetras has a bright red area on its side, not too far behind its eye/head. It looks as red as blood. It's acting and eating normally.

API test kit numbers:
ammonia - not 0, but lower than .25
PH 7.4
nitrites 0
nitrates 5

I did a 20% water change yesterday after the ammonia tested at .25.

Could this be an ammonia burn? Or something else? I don't know much about tetras but I do know this fish didn't have this red blotch yesterday.

I can try to get a photo but don't have one right now. But big blood red blotch by the gill.

 

How is the circulation in the aquarium and is their enough surface turbulence. Red near the gill is often due to oxygen deficiency.

 

The filter that's in there honestly stinks - it's a Petsmart branded one (Great Choice brand, internal filter) that I bought before I knew any better. I have a new one on order (better brand, better filter) that is supposed to arrive tomorrow and I planned to add the filter media from the cruddy one to the new one until the new bio filter is established.

But in the meantime I can increase the flow on the one I have - it's been set to the lowest setting. So there is turbulence near the filter but not all the way to the other side of the tank.

Should I be doing other things to increase the oxygen?

 

Also - the filter arriving tomorrow is HOB, which I think will help with aeration?

 

You could always add an airpump and airstone on the other end to see if the additional flow/oxygenation helps.