I'm a bit of a newbie and need some help with diagnosing what is wrong with my fish please.

This morning he was fine, but tonight I noticed he was sitting down the bottom and his lips don't look right.

The upper lip seems darkened, and the lower lip the normal colour, both lips kind of look turned in and he isn't opening and closing his mouth much.

In addition his left pectoral fin looks a little red, and he has a small red spot just behind his right eye.

I've moved him into an old tank and water changed the main tank. All other fish seem fine.

We're trying to get photos, but he won't stop moving at the moment after being transported to his temporary home. He's a small barb, although I'm unsure exactly what type. Similar size and shape to the rosy barbs though.

Any ideas??

As mentioned, having difficulty with getting a decent photo, but these two seem to show his mouth which he cant close like the other fish can.

Apologies about the image quality.

Is he breathing faster than normal? Can you see anything inside his mouth?

Well I can usually see how fast they're breathing by the speed at which they open and close their mouth (I'm guessing thats what you mean).

In his case, his mouth just stays open and doesn't shut. Can't see anything inside his mouth.

We're wondering if he got too close to the heater while it was on and hurt himself that way - given that the other fish are all ok... Can fish burn themselves on the heater? It actually looks like he has no lips.

Lockjaw?

What size is your gravel? Is it about fish mouth size? :-?

You can tell their breathing rates by how much the gills are moving I think.

I thought the gravel could be a problem, but it looks much too big to fit in his mouth. Gravel looks to be about a centimetre or so long, where as his mouth is much smaller.

There is more movement in his mouth today compared to yesterday, but he still can't shut it.

Breathing looks to be normal, and he was eating this morning. These two signs I imagine are most important.