Mirrors are accurate reflectors. You can see details very well when viewing a reflected image from a mirror, but that reflection may be made up of only 70% of the incident light. A polished aluminum reflector can reflect 95% or more of the incident light, but the image may be wavy or otherwise inaccurate. We want a reflector that redirects as much of the incident light as possible towards the tank, but we don't care at all whether the reflected image is accurate. Older mirrors can even reflect less than half of the incident light, as the silver coating on the back gets oxidized.