well it depends on what you're looking for. If you want the inside to mimic a natural environment then let the sun shine in. Natural ponds etc have algae and plants growing all over them. You'll just have a constant battle of keeping your viewing glass clean. With that large of a tank the acrylic will likely be 1 inch (2.5cm) or more thick. Not many magnet cleaners work at that thickness, of course you could probably shop around for some specialty magnet cleaners... otherwise you're going for a swim for maintenance
Honestly, what I would think would give you the most help and information, is to find any local aquariums and see what you can do about talking to one of their keepers. Find out about equipment, upkeep and chemical costs etc. I'm sure they have a ton of information that would be useful and much more relative than a bunch of us home aquarium types trying to scale up 50+ gallon tank knowledge and trying to apply it to 500,000 gallons.