You could always make the tank out of something clear to maintain the look, mineral oil is also clear.
1. no, convection will take care of it
2. i think 2 radiators is already too many, i wouldn't add a third.
3. You don't NEED those flow rates. They might be what you're targeting. It's all a matter of what you think you need in terms of flow.
4. Mineral Oil, unfortunately, is not cheap, but on the upside it will never go bad as long as you keep contaminates out.
5. anytime
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