Corydora Swimming Towards Top

Hi everyone,

I'm not sure if I have a sick corydora (although suspect so)- first time tank keeper here I'm afraid.
I have a 54 litre tank, cycled before stocking and has been stocked for about 6 weeks now with no problems. I have false Julii corydora, hengeli rasbora, cherry shrimp and nerite snails- all very peaceful! The tank itself has a sand substrate with a few largish rocks providing hide spots a 'bonsai tree' and egeria densa, I also use an air stone. No abnormal water parameters and water changed once/ twice a week.

2 of the corydora I have seem very small, they just haven't grown to the same size the other Corys are. I noticed one small Cory last night occasionally hanging out in the mid section of the tank. I didn't think much of it as I see them swimming all round the tank sometimes. This morning the same Cory was swimming in the same region and for about half an hour seemed to float to the surface when not using his tail as much, then try and swim towards the bottom and then float back up. After this half an hour he went back to the sand and stayed there, for the rest of the day he has mostly been with the others on the ground.
This evening, he is back in the middle of the water and has been there at least the last 2 hours I've been in the living room. On close inspection both of his barbells seem very short or damaged? I don't know if there can be variation so I'm assuming they are damaged. Another bigger Cory seems to have short or damaged barbells also, the rest look normal.

Apart from that half an hour this morning, I haven't seen him 'float' to the surface.
I specifically chose the tank set up to try and prevent any barbell damage- is there any reason for this or anything I have overlooked? He can swim to the bottom and stay there without much effort..... I thought it may be a swim bladder problem but wouldn't this be apparent all the time? I also thought about oxygenation- there is an air stone and all the others seem ok..... he isn't gulping at the surface at all and isn't swimming very near the surface.

Any advice with more experienced Cory keepers would be greatly appreciated! If he is sick and I can do anything then I want to give him the best chance! The rest of the Corys are acting normally.

Thanks everyone!

I hope he has just taken a big gulp of air, Didn't think of that !
Sand is smooth and hoovered at every change- although think I have beeen over feeding as seem to get a lot of debris from food and plants, so have cut down now.
If I'm honest I never noticed any barbell damage when I got them- but didn't specifically check each one either so it could have been there. One other Cory has shorter barbells (probably damaged also I guess) but all the others have nice long and intact barbells.