No it's right that your water is clean, that's good, I just meant overcrowding, I apologize if I wasn't clear.
Well just a few things that can affect a bettas immune response in a negative way are stress, temperature change, injury, water chemistry change, transferring/moving, and water quality.
It sounds like Tom has had all of these changes in a few short days here, so it's possible the injury just put him over the edge :( And we've all made mistakes here so try not to beat yourself up too much.
Wow he is pretty! That looks quite nasty though. That looks like what just nixed one of my girls :/ Is the fluffy stuff on the bottom greenish? Is his stomach white or is it covered in white fuzz? Sometimes when my bettas get injured they shed their slime coats which can look sort of fluffy, but this looks like something else to me.
He seems really stable as far as movement, although he wouldn't eat a worm earlier. Basically, he just sits there and every 20 seconds or so moves around the bowl a bit. If I go over to the bowl he will swim around to come up around side of me after a few seconds.
The green bits are where his pelvic fin was (or what I know the pelvic fin to be from pictures) the long skinny one. The white doesn't seem "fuzzy" per se, rather it seems more like ragged flesh/scales/skin.
Thank you, Tom was very beautiful to me. Very vibrant. No matter what happens I will find the Cory and Danios a good home where they will thrive. I hope Tom makes it but I'm trying to be realistic. Should we get "Tom the 2nd" I'll be very sure to be especially vigilant with making sure I do not have a repeat. Well as much as I can control anyway. It just breaks my heart that he may well be the sacrifice for my learning the hard way.
Hey he's still here...Some can make amazing comebacks. Just to verify now, you have him in a heated bowl with conditioned water and aq salt that you change every day? It is possible that he is shedding scales and slime coat after an injury. They will grow back. FYI, stress coat and a high protein diet will help fins and scales regenerate, if he makes it.
I'm sorry your first experience with bettas has been rough. Like I said, everyone makes mistakes. What's important is that you're gathering information and taking it to heart in case you decide to keep bettas in the future. I'm not sure how familiar you are with the resources available on this forum, but there are certain threads called "stickies" which are automatically the number 1 thread in each section. There are ones on betta care, disease and illness, water acclimation, plants, betta compatibility with other fish...They are just in the corresponding section of the board. Those are great places to start.
It could be columnaris or a fungal infection. Other than the fuzz, how is his demeanor? I have only had to deal with columnaris and it has killed both of my fish within 24 hours or less so if he's been fuzzy for a little bit it might not be that. I hope we can get some other input here. For right now, clean warm water is most important.