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That is so sad..I'm sorry.
I had to Uthanize my son's betta a few weeks ago, due to *Pnuemonia and Fin Rot. There just comes a time when it has to be done.
There are instruction to use clove oil, which I found very hard to locate around me. So I did some research on exactly what clove oil was used for...it's used as a topical pain releiver, to stop tooth aches.
The clove oil makes them go to sleep and not wake back up..unless you transfer them to clean water a few moments after.
Instead of clove oil I used regular strength ambesol. I put 12 drops in his quarentine tank. Several minutes later I checked on him...he looked to be just in the same position he was when I left..floating close to the surface by his thermometer, but no fins were moving (he wasn't on his side..he was perfectly right side up)..but when I lightly touched him..he did not move, just kinda sunk a little (still didn't flip over). He didn't suffer at all, just passed in his sleep.
*For those wondering how the fish got these ailments, my son's room gets kinda cold at night (because my mother turns the heat down at night, to save on gas cost) and he had fed his fish and not put the tank cover back down..I didn't notice til the next evening. He caught pnuemonia from gulping the cold surface air (his tank water is kept between 80-82* and at night it gets well down to 60* or even lower, it got really cold out that week), I couldn't do anything for it really (that I have found directions for) except add salt and keep his water changed alot. He then developed the fin rot..he was skinny, lethargic, wouldn't eat,...he was just too far gone, so I did what I thought was right and put him out of his misery so he didn't have to suffer any longer.