Is my fish sick or injured?

It it was beaten up the damage should be visible. It is very easy to see damage on fish. If I was you I would take some of the water in to get tested. Most good pet stores will do it for free. If you have a local place that specializes in fish that would be best.

Goldfish are really poopy meaning it’s important to keep an eye on water quality. If the water is getting bad it may be effecting the molly. And sometimes when everything is perfect, fish just die from time to time.

@johnpowell thats not necessarily true about it showing. And goldfish are notorious for randomly getting aggressive but they dont really have sharp teeth at all so they dont leave much of a mark. I would definitely agree though to get the water tested first. That should be the first thing you do whenever you notice any problems of any kind in your tank because 9 times out of 10 its related to water quality.

Do you put aquarium salt in your tank? If not I would start it will help the fishes immune system greatly and ime even though mollies are easy fish they tend to randomly die from time to time. Aquarium salt reduces this greatly.

How often do you do water changes on your tank? You should be doing about 10% once a week but possibly even more depending on the size of your tank and current filtration because as @woodcutter said, goldfish are rather dirty fish.

Another question would be what are you keeping the tank temp at? Goldfish generally like colder water where the molly likes 75–80. If the water is cold for the goldfish than this could impact the mollies health. Really these fish shouldnt be together in the first place.

Goldfish like cold freshwater and a pH around 7.5

Mollies, while often sold as freshwater fish, arent. They come from alkaline brackish environments. As stated above the aquarium salt will help a bit but not really in the long run. What is needed for mollies to do good is a higher salinity. I usually keep them around 1.009 (specific gravity) temp 75–80 and pH closer to 8.

@uberbatman thanks for the info. I have a 10 gallon tank; I change 10% of the water every other week. I have a heater in the tank, but I do not have a thermometer…so unsure of the temp. Are there other fish that would do well with my goldfish besides other goldfish? How much salt do you add and how often with my size tank?

Honestly ive always advised people to keep goldfish on their own. Theyre quite dirty and dont really play nice with others that and the water condition thing I mentioned above. There are however a lot of really nice goldfish out there but you are limited due to tank size. As for the salt are we talking to make it brackish for the molly or just the aquarium salt. I believe API aquarium salt is something like 1 tsp per 5g but im not sure, it def says it on the package.

It could be a few different things. I have to ask do you have a heater in your fish tank. I don’t think it would be this, but gold fish don’t need a heater molly/other tropical fish do. Yet it is more like likly the fish’s swim bladder is messed up. It is a more common thing in salt water fish when caught and brought to the surface. It can happen in fresh water fish as well. Just like human organs stop working animals and fish’s organs can break down too. One more thing i can think of is yes he got beat up he should not be housed with gold fish. But i agree with the other person, you would see some sing of a fight. Honestly it is most likely his swim bladder; it is what keeps them swimming upright. You could all ways find a local fish store. Not a Petco or Petsmart, a store that specializes in fish or fish and reptile. They are very knowledgeable and might have a answer for you. Hope it helps good luck.