Betta sudden decline and death
I've had this betta since June of this year (2020). On 12/5 I added 6 cories, but 3 have died with no explanation as to why. One died on 12/15, one on 12/16, and this morning (12/17) I found my betta dead as well.
On Saturday 12/12, I noticed a lump on the betta's back that was the color of his scales, it quickly turned white but he was still acting normal. Yesterday, he was swimming with one fin clamped to his side and was very lethargic. The clamped fin looked normal.
This morning, I found him dead, and when I removed him, the fin that looked normal yesterday had turned black and rotted away, and his top fin looks similar. I unfortunately don't have any pictures from yesterday, but I have one from Saturday and have also added some of his body.
Recent Changes: 6 cories on 12/5, 3 remaining - all acted completely normal up until their deaths. Added some elodea around the same time. 25% water change yesterday after 2 weeks. Found that filter has been working but not full capacity and replaced motor.
Tank: 10 gallon tank that's been running for about 6 years, no ammonia or nitrates (liquid tests), other parameters are from test strips but no nitrites and pH is 6-6.5. I keep the temperature around 78F. I have an aquaclear 20 filter with a sponge prefilter.
There are obviously a lot of things going on here, but I don't understand the incredibly rapid decline. He was acting perfectly normal 2 days ago and now he's dead with his fins rotten away. How can I prevent this from happening in the future? I don't have the ability to run a whole quarantine tank.