I'll have 3 Iwakis for my 160. I mentioned above that two of them are feeding my DT. The third is for my mixing station. It is way overpowered for my mixing station but allows me to cannibalize that third pump for my DT.
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Every situation is going to be different as well as individual experiences. My tank was in my unfinished basement. It was a holding tank while I was redoing a display in my living room. It was February and temps in basement in winter run in the low to mid 60's. While we were away we lost power for a short time. Mag 12 pump never came back on after power outage. Heater in sump kept the sump warm but with no circulation in the display temps dropped and all my sps died. Luckily I didn't lose any fish. Not sure how many days it was out but damage was done. This was about 4 - 5 years ago and I didn't have a net module hooked up to my Reefkeeper. I now run a Profilux 4 so I can constantly monitor while I'm away. As a precaution on this new build 2 pumps are being used on two separate circuits.
As for separate circuits my current single return pump display had a gfi issue while I was away this summer. Gfi tripped three times while I was gone. Not knowing initially what was causing loss of power I had my tank sitter check the gfi and luckily that was it. Gfi was going bad which can happen. A second pump on a second circuit would have been nice in this situation. Murphy's Law.....never know when you'll be a victim.
Not to make this a thread about return pump reliability but this was the second time I had a mag 12 have an issue while I was on vacation. First time it developed a short and kept tripping the gfi. Luckily my dad was my tank sitter at the time and noticed it. I did not have a spare on hand (lesson learned) so I had to drive 5 hrs home, run to lfs to get a new one and 5 hrs back to vacation spot with the family. We only go on vacation twice a year and 3 times I've had return pump issues in thirteen years of reefing. Twice was Mag pump issues and once it was the gfi. Go figure! No more Mag drive pumps for me.Interestingly the Danner Mag Drives have been noted by some as the most bullet-proof pumps. I think even with reliable pumps, it is best practice to have two online at all times (especially if there is a chance you are away for days at a time). I think this has less to do with current state of return pumps, and more to do with the idea that your return pump is the heart of your system. Without it, you cut off circulation, filtration, aeration, and heat. Very few points in your system can cut off that much life support with a single failure.
A dual return pump setup, along with a systematic approach and built-in redundancy where possible help put the odds in the favor your pets and investment when problems occur.
Not to make this a thread about return pump reliability but this was the second time I had a mag 12 have an issue while I was on vacation. First time it developed a short and kept tripping the gfi. Luckily my dad was my tank sitter at the time and noticed it. I did not have a spare on hand (lesson learned) so I had to drive 5 hrs home, run to lfs to get a new one and 5 hrs back to vacation spot with the family. We only go on vacation twice a year and 3 times I've had return pump issues in thirteen years of reefing. Twice was Mag pump issues and once it was the gfi. Go figure! No more Mag drive pumps for me.

