I’ve had this same thing happen before about 2 years ago. When I got up this morning, I saw that my 8 month old oscar, Goth, was very sick. This is what I noticed:
Fogged eyes
Scales very light colored and fuzzy looking, pinecone-ish
Swimming aimlessly
Floating on his side
Bobbing his head above the water

I immediately moved him to a quarantine tank. I observed within the next hour that he continued to lie on his side, float for long periods of time, use only one fin when he did move, and he nudged the corners of the glass with his nose.

I did a 75% water change on his regular tank and dropped in some anti-fungal dissolving tabs. I did this because this could be dropsy, but I’m not sure. The only information so far that I have found on dropsy symptoms was the bloated belly and pinecone like scales. I haven’t noticed if he was bloated but his scales do have a resemblance to a pinecone.

His tank mates are a very large sized pleco and 3 tinfoil barbs. They share a 20 gallon tank. (Yes, I know this is a very small tank for these fish but we won’t be able to move them to our larger tank until we move.) They have a regular carbon filtration system. We do weekly water changes due to high levels of ammonia because of the pleco. I did a levels test and the only thing wrong with the water besides ammonia is that we have very hard water. We feed him cichlid pellets, which he seemed to like a lot.

In the past few weeks we have noticed that he had a lighter strip of scales high on his back on both sides. We thought this was because of ammonia levels.

The only thing that we have used lately in his tank as far as medicine was Melafix. We were using this because we hoped it would help with his lighter scales and one of our tinfoil barbs had lost a few scales.

We haven’t noticed any nipping, besides our pleco sometimes corners him and tries to suck on him… Weird I know. But, Goth always swims away before the pleco can do anything

I thought I had more information to write but I think I may have forgotten some since this is the second time I’ve had to write this. Apparently I wasn’t signed in when I tried to post this last time.

I have a feeling that this is dropsy but I’m no expert. If someone knows for sure that its dropsy or someone has an idea of what this is, please let me know. It won’t help Goth now because he died a few hours ago, but I would really like to know what this is so I can prevent it in the future, since it has happened twice now.

Thanks,
Jenn

Very sorry to hear about your Oscar. Okay, firstly, dropsy is only a symptom of something else that is going on.Because it has a symptom and not on actual illness, it is extremely difficult to treat. In most cases, fish that have dropsy do die. Fish that have dropsy normally have bloated bodies, protruding scales and sometimes bulging eyes. I've got to be perfectly honest with you, I couldn't possibly say why your fish died, or what it had. The only way you could know for sure is to have tests done on the body. I would imagine that you have now disposed of the body. Even so, having an autopsy down on the fish is not as straightforward as you think, you've obviously got to find someone who can do it.

The problem is, you could go on this forum, and countless other forums and everyone is going to give you their opinion of what killed your Oscar. I guess it could have been some kind of internal bacteria infection..but like I said, the only sure way of finding out what killed your Oscar was to do an autopsy