Hi guys. Ive recently added 5 corydoras to my 55g tank. Its been cycled with 3 fish 2 gouramis and a horse face loach. There is now 7 as i removed the dwarf gourami. All the corys were doing fine until today. One of the corys has stopped swimming around like he has been all week and is just sitting at the bottom and only moves occasionally to find a new spot. Ive tested my water and ammonia and nitrites are at 0 and nitrates at 10ppm. Im doing a big water change tonight (40%) but was wondering if this is normal for a cory or if hes ill? He shows no visual signs of illness. Please help?!? :-s