White Cloud Minnow not eating

[serbusfish]

Today when feeding my fish I noticed one of my White Cloud Minnows was hiding out at the back of the tank and not taking any food. It is smaller than the others which I always thought was odd, but ive never noticed that is wasnt eating until today. I have 5 minnows in total, is there anything that might be causing this behavior and what should I do about it?

[j250nyahweh]

Is he smaller but still proportionate or does he looked malnourished, like with a sunken in belly?
How long have you have him?
Do you have any other fish with them? 5 minnows might not be quite enough for him to feel comfortable, how big is your tank?

[serbusfish]

Is he smaller but still proportionate or does he looked malnourished, like with a sunken in belly?
How long have you have him?
Do you have any other fish with them? 5 minnows might not be quite enough for him to feel comfortable, how big is your tank?

He hasnt got a big belly like the others do, but it isnt sunken in, its just sort of flat. Ive had the minnows for around a month now so surely he must be eating something?

Yes there are also 4 Black Skirt tetras in the tank (never seen them bother the minnows). The tank is 27 gallons.

[j250nyahweh]

The one's with the big bellies would be the females, and he may just be the lone male, but what would you say his overall shape is? Is it "football" shaped like this ><> or more more elongated, like this >=> (if that makes any sense)
If it's like the second one he's most likely malnourished and likely not eating because he's not acclimating to his new home. In those cases there's not much you can do.
This happens and is kind of to be expected at times. I have around 30 white clouds and of those 2 or 3 have not made it past a month or so. The rest are perfectly healthy.

[serbusfish]

The second one, he isnt fat at all, I'll try and get a pic.

EDIT:

Hmm when you see it on its own it does look like the first example, but I assure you he is much smaller than the others.

[j250nyahweh]

This probably should've been one of my first questions, but is your tank cycled? Can you test the water parameters, Nitrite, Nitrite and Ammonia?