+1 to almost all of the plumbing advice given so far!
...now my $0.02. :read:
Concerning The Flow
To start with, unless your tank rim is only about 3.5 feet above the pump, there's no way you can expect 750gph from this pump - certainly not after that circuitous plumbing route and ensuing reduction to 1/2" PVC.
It's
rated for 500-600 gph at typical medium sized aquarium heights - before accounting for plumbing inefficiencies. A QO5000 should get you into the 900-1000 gph range, which might net you 700 gph or so after plumbing. Not sure how big your system is, but for reference a 4000 is supplying plenty of flow for filtration for my 100 gallon system and that's 50% (or so) of the return to each display. (Tunze nanostreams provide flow in the displays.) YMMV of course.
Concerning The Plumbing
In most cases I like hard PVC or corrugated tubing for drains and vinyl for returns.
I would highly recommend at least bridging the pump to the straight run up the back of the tank with vinyl tubing - eliminate as many elbows as possible with long, sweeping curves. Also eliminate the 1/2" plumbing altogether.
Also, upgrade to
1" insert fittings and tubing for best performance. Your pump comes with 1" Thread-to-3/4" Insert fittings for the input and output. 1" can be challenging to work with as you note (as well as a little pricey). Make the tubing's "built-in" curve work for you if at all possible - cut it and use an insert-to-insert fitting to connect the pieces back together in the necessary orientation if required. Make one curve of tubing per existing PVC elbow, in essence. Soaking in very hot water will temporarily make vinyl tubing a lot more pliable - helpful during assembly sometimes.
Short of converting to tubing, try replumbing your hard PVC design using two 45º's, or
sweeping 90º's, in place of your existing 90º elbows. This will reduce the added back pressure significantly, if not completely as tubing could. I don't like Flex PVC, FWIW. Unless you're dealing with a scenario where you're above 1" tubing, I would stick with vinyl tubing. (Sometimes even above 1" I'd choose vinyl.) Another reasonable option if you want to keep hard PVC and those wicked 90º's is to upsize your plumbing to 1.5". Use a 1"-to-1.5" bushing on the pump and re-do the same design you have, but in all-1.5" PVC.
Lastly, you can eliminate a whole 1 foot of head pressure by eliminating the tubes that descend into the display tank (and the associated back-pressure) and just return the water at the water surface - ideally with the openings slightly above water.
Concerning The Pump
Concerning Quiet One pumps, I've got to differ with some of the nay-sayers.
I've run a QO2200 for years as my original return when it was a ~50 gallon system. Upgraded to a QO4000 return when I added a 50 gallon display (2nd display) to the system and teed the return flow. 2200 ended up replacing the Mag7 that I had been using as my skimmer pump (AquaC Ev90) and I saved a lot of watts in the process.
While no pump is perfect for all scenarios, generally speaking these pumps are dead silent and strong. Unless you haves a very-abnormally tall tank, or have your sump in the basement, they should make a great solution in most scenarios. gph/watt, these are some of the best - and best priced - pumps around IMO.
To compare, a Water Blaster 3000 will deliver ~
750gph@37 watts for $180 and a 2000
500gph@22 watts for $138. QO2200 will do
594gph@45 watts for $60 and it has more lift, comparable to... A Mag7 which will do
700gph@70 watts for $80. Similar with the higher flow models. (I like all of these pumps, FWIW...Mag's are very tough in dirty/rough conditions, Blasters are very efficient in heat-bound tanks....where possible I use QO's though.)
Concerning The Noise
The pump didn't start making noise until you positioned the pump so the drive unit (magnet) is above the impeller. A lot of pumps seem more susceptible to rattle a little in this orientation, likely due to a small bubble being caught in the impeller chamber. It should not make that noise when running normally - through the beginning of the vid it sounded pretty normal.
If this happens when it's running in a normal position...
Is there any chance there's detritus collected in the sump near where the pumps is? Tank water is used for lube, so a tiny grain of sand or bubble is all it can take to make a little rattle as the pump gets thrown off its rotation. How old is your 5000? Looks like an older model. More than a year old, at least - similar design to my 5+ year old 2200, in fact, as they've changed the housing and impeller design since then. I have sent my pumps in to Lifegard once for "preventive" servicing - worth the one-way shipping and minimal expense in parts (few bucks in by case) IMO if the pump has significant mileage on it from prior ownership. Even including shipping I think I was out less than $40 for two pumps and got a replacement impeller for the 2200 plus some o-ring lube and an extra impeller. (Lube and 2nd impeller were elective by me and probably represent half of that cost.) You still have your old return to use for a couple weeks right?
If you bought that 4000 new and have the receipt, you might just ask for a replacement pump from where you purchased it. Or, it's got a 3 year warranty so Lifegard will take care of you if necessary.
Hope this helps!
-Matt
