My Peppered Cories Keep Dying

A few weeks ago I lost a peppered cory from my tank, I found it unusual, but did not think much of it as it appeared to be only one. I don't know what happened to it, as my tank is heavily planted, and by the time I found it much of it was gone. Then, a week or so ago I noticed another acted odd, laying on its side, breathing slowly. This one was one of my oldest cories, so I figured probably old age (this one is at least 4 years old). While he is still alive, I noticed one of my smaller ones acting in the same way. Laying on its side, shallow breathing, then swims off a bit, before stopping and laying on its side.

Everything else in the tank is fine, there are fish in there that (IMO) are nowhere near as hardy as the peppered cory, and do not appear affected (Angels, Otos, Black and Regular Neons). There are no new inhabitants in the tank, the only newest addition was some plants (which had snail hitch hikers) but they are also in another tank, and the fish in the second tank are also doing fine.

So, what is happening with my cories? I will try to monitor them, but so far the only symptoms I see is listlessness and laying on their sides, they are not laying straight though, but curled a little. I should add though that they are swimming fine, its just when they stop they are not straight.

Any ideas? Thanks.

No change to diet, the thing that confuses me most is that literally nothing has changed recently in the tank. Diet, tank mates, water change schedule etc are all much the same as they have been for at least a year.

Water parameters are 0, 0, 20. In at least the past 12 months the only thing that changed was the addition of some new plants (also in another tank with no effects on the fish) and some removal of plants. The only thing affected by whatever it is appears to be the cories, and whatever it is appears to be somewhat long lasting. The largest cory showing symptoms has been like this for at least 3 weeks now.

Anyone else have any idea what this could be? 2 of the smaller ones died recently, and the large one is still hanging on. I did notice that his stomach is horribly sunken in, but when I drop food he actively swims looking for and (as far as I can tell) is eating the food.

Last night after watching him for a few minutes, I started wondering whether I should consider putting him out of his misery. He does not appear to be showing any worse symptoms, but at the same time does not look happy.

Found another dead fish today, this time it was an otocinclus though, hard to tell if it was the same that is effecting the corys though as my otos are hard to spot except at water change time when they come out.

Trying to go through what may have contaminated the tank. New driftwood went in around early March, and some new plants, no changes since then, no current visible effects on angels/tetras and all fish are eating fine.