My corydoras keep dying

So i have been having problems for a few weeks keeping my bronze and albino corys alive. All the other fish seem fine but noticed my corys are more lethargic and seem to be flicking of the sand and then lay down and die. There is no white spot visible and like i said other fish are fine. I took some water to my local petshop who tested it for me but said everything was fine apart from my gh was low. I changed the water and changed my gravel to sand and took another water test to another petshop to test for me. They told me my ph was massivly low so bought some oyster shells to bring up, i put them in my tank and did a ph and nitrite test which both come back good levels yet i have lost 2 more corys. Can anyone help me i dont want to loose anymore fish but dont no whats happening to be able to sort it. I have attached a photo of my tank and also of the tests i did at home any advise will be great. Thankyou

Well I’m thinking possibly ph shock could have killed 2 more if I’m reading correctly. If all values are good, ide wait til your ph has fully stabilised before adding anything more to the tank. Not a great answer hopefully someone else can give you a better answer

Hi there thanks for reply i actually think my ph was stable before adding them, when i got the second petshop to test for me he said it might not be accurate as i took the water in a plastic bottle and not glass and said that might affect the reading, so when i got home i did it again and ph was 7.5 its not dropped or got higher its stayed the same. How long does it take oyster shells to raise the ph?

The blue test is ph and yellow is my nitrite

I personally wouldn’t worry about that ph, do you know your kh and gh?

I also wouldn’t listen to the pet shops plastic bottle theory lol

Apart from the first petshop telling me my gh was low he didnt give me numbers just said low and the second one only seemed to test the ph which came back lower then 5 but he said it might not be accurate as i took the water in a plastic bottle so when i got home i did another and my ph is 7.5. He didnt check anything else i have bought a liquid tester for all today should come tomoz for me to check it all myself. Just find it strange how my other fish are fine just my corys. And i had a vampire shrimp which died the other day despite it walking round looking healthy day before.

The corys come to life when i feed but not long after they have finished eating there back to been sluggish and twitching, but none of my other fish are twitching. I had 9 corys 4 bronze and 5 albino and now i am down to 2 bronze and 3 albinos

Ide test your gh and kh and put it back here, if you have low kh then your ph won’t move much either

Ok as soon as my tester comes tomoz i will test and put on here will oyster shells work with kh?

If the kh gh is low what do i do to correct that just water changes or do i need to add something

If my gh is low can i raise it by adding epsom salt?

Kaylab2021 said:

If my gh is low can i raise it by adding epsom salt?

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I wouldn’t be worrying about anything really until you know the value then we will advise then

Ignore pH, GH, KH etc for now. It's unlikely to be the cause.

It's not strange that the other fish are okay while the cories are not, since cories (and shrimp, so losing the shrimp sets off alarm bells too) are much more sensitive to poor water quality, and are most likely to be the first affected.

Did they/have you tested for ammonia/nitrites/nitrates? How often do you do water changes, and what percentage of the water do you change? Do you use a declorinating water conditioner, and which one?

Coukd you place pics of your corys. What are they fed?

Hi there i change water about 25% maybe a little bit more weekly and then fill back up with stress coat + api, i feed them a mixture from frozen blood worms,daphnia a few times a week and daily sinking catfish pellets. And i have a pleco so i throw in cucumber now and then which they eat to, they also like to eat at top when the other fish eat. I have done a full test today i will add photos every thing seems good, i just cant figure out how to do the kh and gh i have added 5 drops to kh to turn yellow? And 12 to gh to bring to dark blue is that right? I will also add videos of the fish iv managed to video so far, some are still flicking now and then but not managed to catch that on video . Thanks