How long ago did you clean the filter, substrate and wipe the glass down?
I would do a big (75%) water change and gravel clean the open areas of sand every day for a week. You don't have to move the plants, just gravel clean a few inches from their base so you don't disturb their roots.
I would clean the filter and wipe the inside of the glass down.
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Where did the damage around the head come from, the gravel syphon or was it there before the fish got sucked up?
Is it missing the top part of the tail?
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If it wasn't caused by being sucked up then the cream damaged area around the head and face is an infection (probably protozoan). Treatment would be salt (2 heaped tablespoons per 20litres/ 5 gallons of water) or any medication with Malachite Green (aka Victoria Green) in. You keep the salt in the water for 2 weeks. This level of salt is fine for Corydoras.
If the damage on the face and head was from the syphon, then there is something else going on.
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Corydoras and most bottom dwelling fishes tend to live longer with Mycobacteria (Fish TB) than other fish, and Fish TB kills the host fish randomly. You might lose one fish every few months. They don't normally start dying off within a few days to a week of each other. It's been two weeks since the first one died and I would be looking at a protozoan or bacterial infection but not TB at this stage. When fish die in close succession, it's usually protozoan or bacterial.
While Mycobacteria is a bacteria, it is a very slow growing bacteria that can take 6 months to 3 years to kill an average sized tropical fish. Most other types of bacteria start to kill fish within 24 hours and usually within a few days of there being a noticeable issue.
External protozoan infections like Costia, Chilodonella & Trichodina can live on fish for months and not show any symptoms. However, when their numbers get out of control they can appear as a cream, white or grey patch or patches anywhere on the fish and will kill fish within days to weeks of symptoms appearing. These can be treated with salt for 2 weeks, or Malachite Green, or copper.
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Gary is correct about Fish TB (Mycobacteria sp) being everywhere. When my fish had it I rang around and emailed shops and importers all over the country and they all admitted they either had it in their tanks now or at some stage, or thought they had it in their tanks. The problem with this disease is it comes from the Asian fish farms and the fish shed the Mycobacteria cells in every tank they get put in (fish exporters in Asia, fish importers in Australia, pet shops, home aquariums). The Mycobacteria can live for years out of a fish and Paul from Fish health at the Department of Agriculture here found live but dormant Mycobacteria under rocks in a dry river bed. The river had been dry for 8 years so the bacteria can survive for at least 8 years out of water, albeit under a rock out of sunlight. They can do this because they have a waxy coating over them and it stops moisture loss and inhibits or stops medications getting to the actual Mycobacteria cell and killing it.
There are no symptoms for fish carrying early or even middle stages of Mycobacteria so you can't tell if they have it. The only time they show symptoms is just before they die. Common symptoms in rainbowfish, barbs, danios and other long slender fishes include:
swell/ bloat up overnight, stop eating, do a stringy white poop, breath heavily at the surface or near a filter outlet, die within 24-48 hours of showing these symptoms.
There's no cure for Fish TB and if you think the fish have it, you should get a fish vet, normal vet who is willing to necropsy the fish, or a university biology lab to open the fish up and look for granulomas (lumps in the organs that are caused by the Mycobacteria) and send tissue samples off for culturing to find out what species and strain of Mycobacteria it is.
So there's no cure and no way of finding out if the fish have it unless you buy 100 fish and send some off for testing. You would need to keep the remaining fish isolated from everything else you have and use separate equipment on their tank for two reasons.
1) If they are free of Mycobacteria, they remain free of it.
2) If they have Mycobacteria, you don't want it to spread to your other fish.
Wild caught fishes are much less likely to have fish TB but can contract it if they are put in tanks that have had diseased fishes in previously.
If it is TB, you can either wipe out everything in the tank and start again. But you could end up buying new fish with it and have the same problem in 2 years time. Or most people just live with it in their tank and try not to add new fish. When the last fish dies they disinfect everything and start again.
There's a couple of posts on the forum about Fish TB and how it affects people and fish. I don't have the links currently because I am using my new computer and haven't transferred the links across yet but if you do a search for Fish TB by Colin_T, or even just Fish TB, a couple should pop up. Barry Tetra? made a post about it too and I responded in that post.
If you have any open wounds (cuts or scratches) on your hands or arms, don't put your hands in the tank or get aquarium water on those wounds because you can get a localised infection that will require 6 months of antibiotics to treat. Either wear rubber gloves or just avoid sticking your hands/ arms in the tank until the sores/ wounds have healed.
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