Old age or disease?

[Muuchi]

My favorite molly unfortunately passed away today and I was wondering if he passed away from old age or a disease so I could treat the aquarium with the correct medicine. I think he was around 2 years old and his scales were duller then I first got him. For weeks he would always rest on the substrate after he eats and he wouldn't swim around much. Yesterday when I went up to feed my fish, he would eagerly come up but he wouldn't eat anything which I thought was weird because I thought old fishes would just be more lethargic.
I had a fish that died from septicemia but I put him into a hospital tank. I fear that it might've spread before I put him into the hospital tank or maybe I'm just paranoid.
Water parameters:
Ammonia- 0
Nitrite- 0
Nitrate-20
pH - 7

Tank size - 50 gallons

Now that I typed this thing out I think he died of old age instead of a disease but I want to check with you guys to make sure.

The first picture is when I first got him and the second picture was yesterday

 

[Colin_T]

Possibly old age. Unfortunately we don't always know how old the fish are when we get them and if they haven't been kept in ideal conditions leading up to you getting them, that can shorten their lives.

The fish looks like it has a bit of excess mucous, which can be from stress or poor water quality.

Mollies need hard water (GH above 250ppm) with lots of minerals, and a pH above 7.0 to thrive. If the water is too soft they die prematurely. If the fish had intestinal worms or gill flukes, that would reduce its life expectancy too.

I wouldn't add any medications unless other fish act the way that one did.

If you want to reduce the chance of the other fish getting sick, do a 75% water change and gravel clean the substrate every day for a week. This will dilute the number of disease organisms in the tank and reduce the chance of other fish catching anything (assuming there is anything to catch). You can also clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn. Wipe the inside of the tank before doing a water change. this will remove any biofilm, which often harbors harmful bacteria and protozoa.

Don't add any new fish for at least 1 month after a fish dies.