If you look at the blue acaras in both pictures, the fish in the first picture (with the swordtails) has its dorsal (top), anal (bottom) and caudal (tail) fins standing out nice and neatly from the body. Its pectoral fin (the side fin next to the gill cover) is not open and looks squashed up.
On the other fish (2nd picture with the tiger barb) the fins are not standing out nicely from the body, they are closed a little bit and sitting closer to the body. These are not normal. His pectoral fin is also clamped up.
----------------------
Fin rot is generally caused by poor water quality, eg: water with ammonia, nitrite or a high nitrate levels, and lots of gunk in the gravel and a dirty filter. The quickest way to treat it is to do a 75% water change and complete gravel clean each day for a week. Filters should be cleaned at least once a month.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If you treated the whitespot properly and for the correct amount of time it should be gone now and not be an issue.
Most fish medications can cause health issues, especially if they are over dosed, and it can take several weeks and several big water changes before the fish fully recover from the chemicals used to treat them. However, the swordtails and tiger barb look ok so it might be stress from having 2 male acaras in the same tank and they are arguing a bit.
Depending on how often you do water changes and gravel cleans, I would do a 75% water change and gravel clean the substrate each day for a few days and see how the fish looks after that. If the fish is still clamping its fins then it might be an issue that needs further investigation.
View attachment 88375
