Red eye marks it down as a roach..plus rudd are usually a bit chunkier and have an upward pointing mouth.. It probably came in as fry 4 or 5 years ago and low disolved O² levels overnight has probably killled it.. Carp can manage with less..but youd be best advised to up your aeration and surface agitation..
There can get sun burn marks mate and it you set a shade poll / patch/ sheet /up there will follow the shade all day. 20 years I have had koi but never had time to watch them and nice rud I would say too