1700 liters is actualy around 450 US gallons (1 US gallon = 3.7854118 liters) so it's not 538 gallons but still quite a large tank.
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that will handle the majority of your filtration naturaly. But you will probably still want a skimmer, I hear good things about the AquaC skimmers (but I don't know if those are available in your neck of the planet
) For your size system you might even consider the cost benefit of building a DIY
skimmer, they're not too tough.
And I would agree, for that size tank 2x150w MH
likely won't be enough for many corals you'll want. You mentioned your tank would be 6 by 4 by 3 (Assuming meters) would that be 6 long, 4 wide and 3 deep? a 3 meter depth is very deep.... wait.. i just did some math... 6x4x3 meters is 7200 liters... are those measurements in english feet? if so then your tank volume would be closer to duaumun's 540 us gallons
anywho... 3 feet is still somewhat deep. the bottom of your tank will not have very strong lighting. What i've seen on some overviews of very large tanks is they keep them wide and long, but rather shallow and place a 250w MH
for every 3 or 4 square feet.