Sick Fish Jumping out of tank

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Aquarium Advice Regular
Nov 15, 2021
I got 3 new fish yesterday from a neighbor who was giving them away. There's a molly and two neon tetras. One of the tetras looks super unhealthy so I didn't want to put them in with my healthy fish in case they could infect them, so I put them in a little tank I just set up. Its a 1.5 gallon tank, which I know is too small for long term but since I'm only planning to keep them in there until I think the other ones are healthy and the sick one improves, I thought it would be fine. Tonight though, I took the lid off to feed them, and the sick tetra jumped out and landed on the floor. I took the net and got him back in the tank right away but I think he might do it again if given the chance. I looked it up and I think its because of stress and low dissolved oxygen. I mostly am just wondering what people think about putting the unhealthy fish with my healthy fish because I think it would really increase the unhealthy fish's chance of survival and minimize his stress, but I don't want to harm my healthy fish. I could also put just the molly in or something, but what do you guys think? Also, I put aquarium salt in the quarantine tank because I read that its a pretty general cure if you don't know exactly what's wrong but I won't be able to continue treating him in the big tank because I have a pleco in there and salt is bad for them and I don't want to change the water parameters and mess with my healthy fish. Would just being in a big clean tank be enough to help him heal or should I keep him in the small treated tank? I just don't know what to do and if the benefits outweigh the risks. Also, I have some baby fish in the big tank, idk if they are more susceptible to disease if that would sway what I should do at all. Thanks.
I wouldnt put unhealthy fish in with healthy fish. Also if one of the 3 new fish is unhealthy then you cant rule out that any infection has spread to the other 2, but they arent showing symptoms.

Fish jumping is a sign of poor water quality. Does your 1.5g QT have a heater and filter? Is it cycled? Do you know your water parameters? Preferably taken before a water change. Even if its cycled its going to be difficult to maintain water quality with 3 fish in such a small tank. If the QT has a light, try turning it off to reduce stress levels. Do you have a larger alternative? A tote box with an airstone and heater is going to be better than such a small tank. Even a bucket would be better.
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Ok, I just got a 10 gallon tank for them. I haven't put them in yet, but I'm going to soon. It has a filter, but no heater because it's hot here and the water is at 76. I'll probably get one soon anyways so that the temperature is more steady, and I'll need it for winter, but as of right now there isn't one. I also put an airstone in the new tank. Because it's new, it isn't cycled but I added some sand from a well established tank to help jump start it. Since its so big for 3 fish, I'm not too worried about their bioload being too much. I know how to cycle a tank and I have a test kit to keep track of paramaters, so I'm not worried about that either.
The main thing is that I want to find out what's wrong with the fish so I can treat it better. One of the tetras is super skinny, like if you look at it from above its paper thin. The other one just hasn't been swimming a lot and isn't coming to the surface for food, it just waits for it to come to it. I'm not sure if that's normal behavior or not since I just got it. For right now, I'm going to stick with using aquarium salt and keeping the water clean, but if anyone has any ideas of what else I can use to help them, please let me know. Thanks.
I'd be very wary of adding salt. Many freshwater fish have a low tolerance for salt. There are certain fish diseases and parasites that might be cured or ameliorated by the addition of salt, but just adding salt because a fish looks ill is not a good idea.
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