I’ve decided to purchase the Superfish Scaper Tank 90 as my first new tanks after many years without one. It comes with most of what I want (may change filter later), but doesn’t have a cover. Now, where we live the water is very hard and I want to avoid too much evaporation so I think it would make sense to have an acrylic cover. I can order one to cover the complete area or I could leave a gap around the sides. What is best?