Tl;dr: older betta developed dropsy. Is it "worth" treating him at this point? How do I sanitize the tank when he passes, as I'd like to get pea puffers later?
I have a "rescued" male betta that is at least 3 years old, likely older. The first 2 years of his life he lived in a 3 gallon with no heater. I moved him to a 10 gallon with live plants about two months ago. He has always been pretty inactive, even when his previous owners had him, but I noticed about two weeks ago that he didn't seen to be doing well and thought it was old age. He'd be better for a day or two, then back to being unwell. Overnight he started pineconing and I realize now that he has dropsy and most likely won't make it through the night, but I don't know how to euthanize him. My nitrates are at 25ppm after adding ferts today, nitrite is 0, ph is 7.8, his tank isn't gross, I have no idea where it came from. He's in pretty bad shape and I don't know that treatment would even help at this point. What do I need to do to sanitize the tank afterwards? I want to get pea puffers to put in here later on, but don't want to risk them getting it as well.