About the betta: what do you feed him, and how much? One of mine recently went through the same thing, laying on the bottom a lot and looking like he was dying. Moving occasionally to different spots but lethargic. I thought about his end-times, but then, as Aiken says, about what was wrong before writing him off.
I decided to address constipation (and swim bladder disease) first. Fasting the fish for 24-48 hours is recommended. He'd been indifferent to food for a few days so I quit feeding to see how long before he appeared at our agreed upon feeding spot. It was 24 hours before he showed an interest.
A green pea is used for constipation. Run one under hot water until it defrosts in your fingers; pop it out of the husk. Pinch a tiny bit of it to feed the fish. Now, here's something no one has ever mentioned to me about using the green pea cure: even the smallest chip of pea sinks to the bottom, while the fish floats at the top, getting nothing. On my third try he finally noticed it and dived to the bottom, but still did not find it. On my fifth try (!) I finally figured out how to actually get the pea into the fish: a little spoon, the kind they give sample tastings with at ice cream parlors. I lowered it barely into the water line right in front of him with the pea piece directly going into his lunging mouth. We were both trying here. Success! Literally spoon feeding the fish.
Just that one piece. He gobbled it and I could see he didn't like it. After that, no more food. But 12 hours later - he was his old self! Zipping around, full of life again. I still waited until he was really hungry, begging for food at his usual spot. It was another 24 hours for that.
I feed all my bettas the same, but this is the only one, the glutton who never seemed to get enough, who's ever had a problem. So now he's on a diet: feeding only once a day. The evening meal is out. And only half rations at breakfast. He begs for more, but lesson learned, as he's acting like his youthful self again.
It might be worth trying this simple, basically free test before going the chemical or euthanasia route.