EDIT: I went to my work place and tested my aquarium water. I found the problem. Everything was good, except my nitrates. My boss asked me if I put in the aquarium salt and I said "No, not without a filter". She got really pissed with me and told me to bring in my fish and she'll flush him. I started crying. My dad did end up coming to check out the pump. At first, he thought the pump was dead, but then when he took it apart, it was clogged with algae. My dad came with me to the store and when my boss saw my dad, she was totally two-faced and was being nice to me. When he left the store, I told her I was sorry and she made a face at me.

Lock please.

I know next to nothing about fish tbh. But I can do some googling and see what that yields?

I googled all night. I work at a pet store, so most of my knowledge comes from there. I bought aquarium salt, but I can't add it to my tank because my pump isn't working. I disconnected something while cleaning it and sliced my thumb on a part. In the meantime, I have my emergency battery operated air stone in the tank. My dad said he'll look at my filter tomorrow.

If you want to google, go for it :)

Oh geeze, well then more than likely I won't be able to find anything you probably don't already know. Is there any chance he might be sick and that's what's making the fins black?

Edit: I noticed your initial post says that "...and feed them a wide variety of foods, which I don't do." Does he need to have a wide variety of food like you stated? Cause if you aren't and he needs it, then maybe this is part of the problem? Also I'm reading something about highly specific water conditions beyond just temperature. Apparently they need a specific PH balance of ammonia, nitrate and etc. Have you checked that your waterparameters are correct?

It also just occurred to me that you may have indicated you don't keep the water at the required temperature either. That could also be an issue.

My water temp is perfect. I checked the heater gauge tonight. It's actually at 77, I think. Hard to tell with calcium deposits on it. Oscars eat worms and other small fish. Whatever they can fit in their mouth, they'll eat (they also have teeth which is why I'm keeping my arm out of the tank haha). We have plenty of variety of frozen foods at work. Brine, Mysis, Bloodworms... I can always buy him some shrimp.

Hmm. I didn't find anything online about PH balance of ammonia and nitrate. Our tap water here has naturally high PH. I read somewhere online that Oscar tanks only need a 25% water change instead of the 50% change. I took out a little less than 25% tonight. I wouldn't even know how to raise my nitrate.

I honestly am not sure what any of that means. But yeah thats what I read on one page. I also just found more information regarding tank size. One page I looked at is saying something about baby oscars requiring a 45 gallon tank, and then something like 55 or more for an adult? According to the page Oscars excrete a high amount of waste and the large tank is require to keep the waste levels from being too toxic for the fish.

Yes, they need a 55g+ for one adult. Oscars are very messy. I know I made a mistake taking Oscar, but my boss wanted him gone.

Hmm. So I'm looking back over your first post, you did say it's just his fins that are black? Because alternatively I have seen some forums discussing black fins like this and when pictures are involved the other posters are saying the fish is just fine. But I don't know how black your fins are. Is the fish behaving abnormally aside from the color change in the fins?

I'd also look at the fins carefully to see if they look damaged at all. It seems that a bacterial infection could also be the cause of the fins turning black in which they will rot off. Some people are recommending changing the water once a week.

Edit:

I'm starting to find next to no new information but I did find this post too, I'm going to quote it exactly:

"If the tip is just edged in black without the disintegration, this can be a sign of ammonia poisoning or an other chemical irritant in the water. Treat both by starting with a 50 % water change to improve water conditions and adding a little salt to their water - 1 tablespoon per 5 gallons of tank water. That should be all that's necessary to reverse color change from an irritant, but you should try to determine the cause - monitor tank chemistry (ammonia, nitrite, and nitrate) or watch medication dosing or other chemicals being used in the same room to prevent this from happening again. It may take 2 weeks for the color to revert to normal."

This was in regards to another person's Oscar's fins turning black. I'm really having difficulty finding any other potentially helpful information on this matter.

Peharps it is fins rot? My betta had it and I healed him by cutting the rot on the fins (very difficult task) and by adding a blue liquid that I found at the pet store (it regulates the water PH and remove impurities). I only use mineral water to clean the aquarium and for the fish itself.

Can you provide actual photo of Oscar? (with closeup on the fins)

It's defiantly not fin rot. Oscars have a habit of having black fins when they're unhappy. When I got him from work, his fins were white. Guess he was happier there, than here.