depends on how much water you have total, how cold you want it and what lights you have on it. but i would go with a 1/4. thats what i have. i do have a 1/10th i used on mine with 3 400w mh 2 175w mh 2 6ft vho 2 4ft t-5 for awhile but it was running the whole time the lights were on. but i did have almost 300 total gallons in my systems.
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