dead corys

please advise - I've had this size for 6 months, upsized after 3 years with 80 litre, so naturally, i bought a few more tank mates from sociable categories at local aquarium shops in UK. sadly i lost 2 baby plecs and 3 corydoras (albino, one young, two from the previous tank ) all around xmas...

This has been a bad week for my tank I have to say 3 bronze Cory died this morning other fish are ok pleco is not her active though starting to think I’m in a down hill spiral in the last 4 days a total of 700l has been changed in the tank and still Cory deaths , sound crazy but is there...