I was hoping jaysee, or someone else with that level of expertise would have gotten back to you by now. Any ammonia at all is cause for concern. Combine that with 40 Nitrates, (50 being the limit in most references I've found, though, personally, I wish they all said 20 for a limit), & your fish may actually be in pain, which would curb their appetites. Corys are especially affected by ammonia, nitrites & nitrates, or any pH swing, that might accompany spikes in those. If it was me, I would do a 50% water change, trying to keep the water you put back in as close to the tank temp as possible, though a degree or 2 higher might be better, making sure to dose well with anti-chlorine before adding your water of course, using Prime especially if you have it; a lot of people seem to like it best from what posts I've seen.