Powerheads keep the watermovement and circulation high. You Ideally want a very chaotic waterflow. for a 75 gallon tank you will want a waterflow of 750-1500 gph
2 or 3 large Power Heads will do it!
Marine fish need alot of water movement, they are used to it in their natural environment and are stressed without it. Corals, sponges, anenomes and other inverts will die without a high amount of water movement.
I have a 37 gallon tank with a flow of somewhere around 450 GPH
Check out my gallery to see how I have placed My Powerhead oppisite the exit of my Canister Filter and both streams meet in the middle and create a chaotic flow.
Hardware: 37 gallon SW
tank and stand by Oceanic Systems, Magnum 350 Pro Canister Filter (half carbon), CPR Bak Pak 2R skimmer, Maxijet 1200 Powerhead, RenaCal 150 W heater, Compact fluorescent lights with moonlights. 40 lbs of liverock, 3" sand bed.
Software:2 Tank-Raised Ocellaris Clowns, 1 Purple Firefish, 1 Electric Orange hermit crab, 18 Blue Legged hermit crabs, 8 or so Nassarius snails, Xenia, Blue, green, and Green Striped Mushrooms.
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Central Illinois Marane Aquarist (CIMA) member.