I wouldn't worry too much of a trace of ammonia if that's even what you really have. Like I said before, with this type of cycling you may not see nitrite ever but that's good! It's surprising you don't see nitrate but you will eventually unless you have a ton plants or obsess over water changes 
. Try to keep them under 20, 10ppm or under is fine in a planted tank...but you really don't have enough plants yet to be concerned. The thing with root tabs AFAIK is they "pretty much" stay in the substrate, not much goes into the water column. I have medium to heavy-ish planted tanks & see 5ppm at most.
Next time you get a good look at the loner cardinal look to see if it looks like the blue line is bent, interrupted, or has a white space between the red & blue. Get a pic if you can. This is a sign that tetras are prone to. It's called "neon tetra disease", look it up for pics. There is no treatment & can be contagious. If it gets to the point where it gets worse you feel it can't possibly be anything else or can't possibly get better, well, it might be time to consider euthanasia (sorry, very very sorry, it's sometimes the best option sad as that is). BUT, before you get to that point post pics & get opinions in a new thread. Just trying to prepare you for what CAN happen, not what is going on with your fish now.
Hopefully it will act normal soon, it's really only been a few days, way too soon to panic. It's good that it eats. Water changes are almost always good & it's a good habit to get into. If something seems wrong, WC; fish not quite happy, WC. Alarming test numbers, WCs!! That (in case you didn't get it) is always my first line of defense. Your tank is small enough that WCs are not hard or time consuming... my smallest tank right now is a 55g & still not all that hard...but my husband often helps.
. Try to keep them under 20, 10ppm or under is fine in a planted tank...but you really don't have enough plants yet to be concerned. The thing with root tabs AFAIK is they "pretty much" stay in the substrate, not much goes into the water column. I have medium to heavy-ish planted tanks & see 5ppm at most.Next time you get a good look at the loner cardinal look to see if it looks like the blue line is bent, interrupted, or has a white space between the red & blue. Get a pic if you can. This is a sign that tetras are prone to. It's called "neon tetra disease", look it up for pics. There is no treatment & can be contagious. If it gets to the point where it gets worse you feel it can't possibly be anything else or can't possibly get better, well, it might be time to consider euthanasia (sorry, very very sorry, it's sometimes the best option sad as that is). BUT, before you get to that point post pics & get opinions in a new thread. Just trying to prepare you for what CAN happen, not what is going on with your fish now.
Hopefully it will act normal soon, it's really only been a few days, way too soon to panic. It's good that it eats. Water changes are almost always good & it's a good habit to get into. If something seems wrong, WC; fish not quite happy, WC. Alarming test numbers, WCs!! That (in case you didn't get it) is always my first line of defense. Your tank is small enough that WCs are not hard or time consuming... my smallest tank right now is a 55g & still not all that hard...but my husband often helps.
