Divider tank is making 1 fish float n one sink...Why?

[ChristinaJean]

So I bought a Betta prolly 2 months ago and had him in a tank that you can put a divider in so you can put 2 in the tank. Well he has been n the tank by himself for those 2 months and I bought another betta 2 days ago and put the divider in the tank then put them both in there and ever since I did that my old betta floats and cant stay at the bottom then my new betta sinks and just lays on the bottom. Now my old betta was fine until I got the new one. Can someone help me please?

[Oldfishlady]

Sorry you are having problems....got a few questions...

How big is the tank, filtration, live plants, how much and how often are the water changes, additives used, substrate, water temp, type of food and how much/how often fed, have you tried any treatments or medications?
Also, can you post a pic?

[zelilaa]

everything she said ^^ MOST important thing to know is how big it is please! (in litres or gallons)