In my opinion the sunlight will be your biggest problem with algae but that may be able to be corrected with a shade cover that could be placed over the tank in the afternoons. As for the agitator I personally would build a dump style wave system for flow. Filtration is going to be your best friend however. For a system that large I would bulds a 275 gallon sump/fuge with a deep sand bed and a good uv sterilizer and a skimmer. I would then add about 4 or 5 yellow tangs to the tank for algae control and some snails. That's just me. I would also look into heating for ponds to see if there is a large enough heater out there or even cattle tank heaters that farmers use for winter time that could be converted with titanium coils.