Algae and microbes are usually more of a problem with outside tanks, due to the increased amount of moisture in the tank. Corrosion is another concern. Believe it or not, today's fuels can actually allow the tank to rust on the inside. The more moisture there is in the fuel, the worse it is. This summer my brother ended up with 3 bulldozers and an excavator shutdown due to contaminated fuel. The farmer's tank had moisture in the bottom, and there was a bunch of rust/sludge in the fuel that plugged the filters on the equipment.
Keep the tank inside if possible, use a filter on the outlet hose, and use a multi-purpose fuel treatment as well.