Corydoras Problems

I have 9 Corydoras in my 29 gallon with a dwarf gourami and 11 x-ray tetras. The Corydoras I have noticed are experiencing some problems recently.
First a few maybe two are losing their tail fins and some have damage to their dorsal fins. I don't know if it is water quality issues but I have been doing a weekly water change of 10 gallons, I did just add a canister a month and a few weeks ago but I ran it with the old filter for a month before removing it. I doubt nipping, the DG rarely goes after or even pays attention to the Cory's.
Secondly I've noticed them breathing heavily at the bottom, they'll swim around fine and are relatively normal but some will just stop and suddenly start breathing heavily. The temp has been high as my ac is broken and the temp will range from 81 to 78 usually dropping to 78 at night and 81 in the afternoon. They aren't swimming up to breath a lot though just staying at the bottom breathing heavily.
Lastly, one corydora that is the smallest of the group lost his barbels no other Corydora has barbels damage just him, he doesn't have problems finding food but the size difference does show some issue eating, how can I help him? I don't know if I got him this way or he damaged them.
The second and third picture is of the two Cory's with dorsal fin damage. The fourth is the smaller Cory missing his barbels. I cannot get a video of the breathing to upload but if I had to describe it the Corydoras gills move rapidly with their mouth moving too and it looks almost like it is trying to chew something.

 

Sometimes corydoras that are 'breathing' heavily are just disposing of the sand they sucked up when they were scavanging, if you look closely bits of gravel and other debris that they don't want will come out of their gills. If they aren't coming up to the top for air then I doubt you have an oxygen problem. They may be nipping each other, I feel like I had this problem too because some of my corys were missing tails, etc. when I bought them and then I thought that some who had no tail and lost it since I bought them, it stopped happening when they grew up and the tails grow back. I have a DG in my tank too so I don't think it would be him causing the harm. What are your parameters?

 

I did a water change around 1 am and dosed prime so I will have to wait on parameters till later today. The tails I doubt are from nipping, but could be possible, they also could of got ripped and aren't healing right. The dorsal fins could definitely be nipping though as that would make sense probably just establishing a pack leader or fighting over a shrimp pellets. Parameters hopefully are better but after removing the hob filters for my canister there was a slight ammonia spike but I'm hoping my bb has caught up by now.

 

Corys don't have to fight for a picking order and won't fight over shrimppellets. Corys simply don't fight, nip otherwise than male being roughly chasing females.

I've noticed albinos often show damaged fins. Think iy has to do with inbreeding aso (bronze in the same tank have no issues).

Could be caused by ammonia/nitritespikes or have a bacterial cause.
I'd treat with a antibacterial med (not Melafix) and think aboit adding Indian Almond leafs

 

I have Indian almond leafs and add some and I didn't think the Corydoras were causing others harm just stating if they were I would think it to be over dominance or food. If anything I would expect my tetra or dg of nipping first. It is possible they may have damaged their dorsal fins on the rocks in the tank. I try to make it damage proof but I could of missed small spots. Just tested my water and small ammonia does seem present and nitrates seem high. The reading to me seem to be 0 or .25 ammonia, 0 nitrite, and 20 or 40 nitrate. I'm in between the readings as they look lighter in person than in the in picture as you can see with the first three compared to the last picture. None the less nitrates are high even though I have plants and just did a water change also there is a chance slight ammonia is present. I'm gonna guess when I removed my hob filter the tank had a minI cycle or ammonia spike and caused damage to my corys but no casualities happened. I think the tank has recovered though or is near recovery but I think another water change will be preformed in the afternoon for the nitrates.

 

The nitrates aren't that high but waterchanges are great for / during recovery.

 

Okay, I though 40ppm was the max you would want present in a tank. I'll try to add an extra water change to help and see if it improves the Corydoras health.

 

Well it's been awhile and Ive done my water changes and even cleaned my canister filter but I don't see improvement or further damage. The tails of the two still appear damaged and the missing parts fins are still missing. They all still move and shift through the sand but will also lay still when they want. The tiny one is still moving but mostly rests throughout the day and I believe he is having a hard time finding food and won't always eat even when it is in front of him. I don't know if any improvement will be seen in only a 9 day notice but I'll still be observing for changes. Any meds I could use as a dip/soak to help with recovery? I don't have a spare tank as I am returning to college soon and won't have room

 

Two pictures of the ones with the most damage to their fins.