Corydoras dying after some filter disturbances, other fish in tank are fine. Looking for advice.
I had two unfortunate back to back occurrences in my tank a few weeks ago and even though things are stable, I still seem to be losing Corys (and only corys). Occurrences:
One of the two filters on the tank stopped working while I was out of town for a couple days. I don't know how long it was out before I got home and unplugged it (max 48 hours). It took me a few days after that to get a replacement. Thankfully I run two filters so the tank just less water flow than normal. I monitored the water parameters and they stayed steady throughout the time the second filter was down.
When unplugging the broken filter, I accidentally knocked an air pump into the tank (plugged in) and didn't notice until I went to replace the broken filter a few days later. I don't know what the effects of this might be (electrical currents??) but temperature didn't spike.
All my corys still seemed fine when I first got home, but I noticed they started to pass that week. The losses seems to stop for a bit, but I'm worried I'm seeing fewer again. I had 14 false juliis before all this, then last week I could regularly find five and now I can only find three (I only found 6 bodies total, but the tank is quite densely planted/has plenty of hiding spots). I also had 6 pygmy corys until a few days ago, and thought they were unaffected, but suddenly I can find only three.
Cory symptoms: The corydoras that are alive always seem fine. One is perhaps a little paler than normal and they've been a little less active, but they're still swimming normally. The one of the pygmies I could find today looked maybe a bit round and reddish in the torso? I don't see any white spots, or weird growths or any other visual changes.
My other critters: All my other fish are fine. No losses in the neon or ember tetras or the kubotai/chili rasboras. No obvious losses of neocardinia shrimp or ramshorn snails. My otocinclus still seem happy and one looks like she's carrying eggs again. The only fish I could convince myself look off are one galaxy rasbora that looks pale (might just be that one's colouring), and a strawberry rasbora that has a weird white bulge that looks more (to me) like a physical issue than a fungal one (video in my post history, I've isolated the fish and it seems healthy so far except for whatever the blob is).
I would very much appreciate advice. I'd like to eventually get some new corydoras so the survivors aren't lonely, but not until I'm certain this is sorted.
Tank parameters: 33G long, been up and running for over a year and this is the first major issue/fish loss, heavily planted, running two Aquaclear 50 filters, GH 8, pH 7.4, Nitrites 0, Ammonia 0, Nitrates ~5, kH 2-3. I top up with RO water and do an ~10-15% water change every 2-3 weeks (tap water treated with API tap water conditioner). I've done a couple extra over the past couple weeks in case it was something in the water.