For anyone else trying to download the NixOS ISO from the website, they don't offer torrent downloads and the connection keeps dropping (for me, at least), I eventually got an intact copy.

magnet:?xt=urn:btih:071B7CBA20B94E5FDC4DAD397DE98D3A05276536

SHA-256 can be verified from https://nixos.org/nixos/download.html