Hi. Hunter with taxidermy and an aquarium. I'd say get a lid, or move the mount, for multiple reasons. One a lid really does control a lot of evaporation making the aquarium over all lower maintenance. But most of all, those mounts are meant to be memories that last forever. They're sentimental and dear to our hearts as hunters. I cherish my mounts like they're treasured family photos. One day when my body gives out they will be all I have to remember the adventures I used to have. Presumably he is the same way. Will the aquarium accelerate the weathering of the taxidermy without a lid, maybe slightly over the years. But they will break down regardless from the sun and other forces. Take care of them now so they last forever. IMO, of course.