Sick fish

Can anyone tell me what that is on this fishies tail?? I haven't been able to treat it properly because I don't know what it is..

If you can post another picture without the black filter on the tail it might show more?

The fish has a bit if excess mucous (white dot) on its back just in front of the dorsal (top) fin. This is usually caused by something in the water irritating the fish. It can be poor water quality (ammonia, nitrite, nitrate or pH), or an external protozoan infection like Costia, Chilodonella, Trichodina.

Check the water quality for ammonia, nitrite, nitrate and pH.

Do a 75% water change and gravel clean the substrate every day for a week. See if it improves. If there's no improvement after a couple of days, post more pictures, and tell us more about the tank (how long it's been set up, how long you have had the fish, etc).
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Thanks for the response! I'm currently doing 10% water changes every other day and the nitrates are a lot better than they were (about 20 or less now). I'm treating for white spots which seem to be getting better I just have no idea what the gold-like/dark bumps on its tail are, and I don't want to put it back in the main tank until I know what it is... That's honestly probably the best picture I can get of its tail, but I can try again tomorrow. I've had my tanks/fish since around April. I noticed the spots on the fish a few months ago and moved it out of the main tank and was never able to figure out what it is.

Here's another image of the fish without the filter.. wondering what those gold/brown bumps are....

If the fish has had the 2 dots on the edge of the tail for a few months and they haven't changed during that time, it is probably just colouration. If it hasn't changed during that time, then put the fish back in with the others.