I had to euthanize my betta, can I leave the tank running for a month or so, preform weekly water changes and have all the "whatever needs a host fish to live" die off? What should I do before it's ready to house new fish, thinking about substrate and plants in there (java fern, stem plants and moss ball) as well as it housing the same sponge (in hopes of keeping it cycled) in the HOB filter. Should I sanitize everything in bleach solution and rinse the substrate or throw it out?
Here's the history of the tank and betta below if it helps.
I had a betta that developed a moldy ventral fin, I added "tetra all in one" tabs, I think two, but stopped after and kept up increased water changes and it was all looking good. A few weeks would pass and saw maybe only 4 ich spots or so show up, I preformed water changes at the sight of anything and we were back to normal and what seemed healthy. Then some time passed and he was at the bottom of the tank breathing really heavy, tested the water all was good, no ammonia, no nitrites and super low nitrates, but did a water change added aquarium salt.. after some time that did not resolve and I had to euthanize, heartbreaking. Now the tank is running with some plants, a moss ball and fluval stratum. I syphoned all the water out (as low as I could) and added fresh with prime of course. I'll add that my fear is that I introduced something to that tank from another when adding a plant. I had gotten a sick glowlight tetra, breathing super fast since I got it but stayed like that for weeks, looked maybe malformed, I never really knew. I've since euthanized that fish in fear of other inhabitants in the tank (this is a separate tank), but all other fish have been doing well for some time. Maybe my betta was old and low immune system (don't know the age we were his third home). Should I be scared of fish TB (possibly from glowlight tetra infecting both tanks), or unlikely the case?
Thank you for reading, I've tried to learn as much as I can and I don't want to be too stress out that it turns me off fish keeping. I really appreciate any input.