Panda cory behaviour?

[the-it-thing-person]

I recently bought 3 panda cories to go in my 54l tank. I already had 6 rummy nosed tetras, and I added an ancistrus at the same time as my cories. The tank had been set up for 3 weeks, with tetras added after 1, but still not going up for food or lookng very red-nosed, but high nitrite delaying any more purchases. Through thourough cleaning I managed to lower the nitrite to a good level last weekend. This morning I noticed 1 of the cories was dead and another was constantly swimming up to the top, staying there for a small while, then coming back down. It's head is pointing up when at the surface, and I am thinking that it is feeding, as last night I put in sinking granules, but a lot were still floating this morning. I'm returning to the shop tomorrow with my guarantee and a water sample, but is this cory's behaviour something to worry about?

Also, if my water is fine, I plan to add a ram and a few cardinal tetras. Is this a good idea or will there he aggression or overstocking?

Cheers