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So i recently bought a 180 gallon used tank that was used for a saltwater aquarium. The person sold me the whole tank and stand for 300$, but the whole tank is just covered with residue and coral parts I believe, and since it's an overflow tank it's in the part that goes to the canister filter. I don't know what to do to clean that part out and the whole tank in general?

What does the overflow look like? Usually vinegar works fine. I normally use single edge razor blades to scrape tougher residue. I've also used a magic eraser on glass, but be aware that it is abrasive.

This isn’t the best picture of the tank, but this is what it looks like.

Yeah, you'll need a lot of elbow grease for this. Just be extra careful with razors near the silicone, as it will easily cut into it. Though if there are any loose seals, you'll have to reseal once it's cleaned up.

what could I use to clean the inside of the overflow part of the tank, because i don't know how to clean it.

I'd first try the things I mentioned earlier. How big is the opening?