Light-filtering curtains are an option. They cut down on sunlight and UV. It doesn't have to cover the whole window: study the spot you're putting the tank and watch if direct light comes from high or low at different times throughout the day. You may be able to do a half curtain to only block the tank.
These curtains can be custom ordered from curtain sites, or buy something too big and take it to a curtain shop or even a tailor to cut down to your exact dimensions.That way you'd still have full view above it.
Similarly, light-diffusing shades, available at Walmart and online, can totally cut off the direct sun or be opened by degrees depending upon need. Hanging one at half height, like you would the curtains, would take care of them while allowing your original setup to remain the same from the waist or neck up. Hope this helps.