Corydora catfish fin rot and clamped fin

[Dracopax]

Please help! My group of panda corys seem to all have a bit if fin rot - one seems particularly ad but all of them seem to have a bit on their tail fin. One of my corys also seems to have clamped fins. The weather has been quite warm recently and due to holidays they were two weeks between their last water change.

Water has now been changed, parameters are:
Ammonia 0ppm
Nitrite 0 ppm
Nitrate 0ppm
Ph 6.5
Temp is currently 24.5 but typically is 23 centigrade.
I've added an airline too to try to help with oxygenating the water

The tank is 90 L planted tank. Half planted, half fine sand

Tank inhabitants are:
6 panda corys (plus a few fry)
4 ottocinclus
3 rainbow threadfins
Cherry shrimp
Amano shrimp
Ramhorn snails

I tried to take a few pictures... they're sadly not the best

Please help! Any advice would be very great fully received - I'm really worried about them

 

[GreenGuy]

nitrate should be at least 5 - how new is your tank, and did you cycle?

 

[DoubleDutch]

Think i see nematods / flukes (on the tailfin)..
I'd treat with Prazipro.

 

[itiwhetu]

How old are these fish.

 

[Dracopax]

nitrate should be at least 5 - how new is your tank, and did you cycle?

The tank is about 6/7 months old and I cycled for 1 month before adding any fish. I added them gradually too

It's got seachem matrix in a oase biomaster 250 filter

I used the API test kit and made sure I followed the instructions. Is there a better test kit to use?

 

[Dracopax]

Think i see nematods / flukes (on the tailfin)..
I'd treat with Prazipro.

Thanks! For prazipro, would I need to remove snails/shrimp?

 

[Dracopax]

How old are these fish.

I've had 5 of the for 3 months. One of them I got almost a year ago with 1 friend (different tank) - sadly the friend passed away shortly after they got home and I read you needed groups of 6 or more ideally so I got a larger tank (the current one)

 

[itiwhetu]

I've had 5 of the for 3 months. One of them I got almost a year ago with 1 friend (different tank) - sadly the friend passed away shortly after they got home and I read you needed groups of 6 or more ideally so I got a larger tank (the current one)

I'm asking this question because as fish age the lines between scales become more pronounce. These fish seem to have distinguished lines between their plates indicating age.