How are water pumps sized?

[bartman]

I purchased a used 125S tank and the pumps do not work. The tank is drill in the bottom and is 20 inches tall. The stand is 42 inches tall. The filter set up is 2 Red Sea 325 canisters and the pumps it came with are little giant 3-MDQ - SC. They are rated at 650gph at 3 feet. I was told this is a flow pump and not a pressure pump....

Whats the difference?
When do you use a pressure pump?
How do you measure the 3 feet?
So am I looking at 3.5 feet or 5 feet?
Can I use a different pump that has less wattage like the Rio 14HF?

I'm so !

 

[FishSeller]

bartman,
Head pressure distance is measured from where your pump is located to the top of the tank (or wherever the pumped water exits). Your little giant pumps are quality pumps, but they do require some maintenence and they can be loud as well as expensive to operate. If I am not mistaken, one pump is a 3 MDQ SC while another is the 3 MDQX SC. One of them is a flow pump and the other is a load pump. I do remember owning the 3 MDQX SC. I was not running a pressurized system, but I usually overkill, so it wouldn't surprise me if I bought a load-bearing pump to simply flow water...
Right now, I have an 80 gal. cichlid tank with a wet/dry that uses a Mag Drive #7. This pump has been used, abused and neglected for years without any major problems. I have it running in-line (not submerged in the sump) and it seems to work pretty well. I'm trying to picture how your set up is working with two pumps. If you want to keep using two pumps, it may not be a bad idea as you will have the peace of mind knowing you have a back up should one fail. The downside is you will be using a large amount of electricity to keep things running which can become expensive (remember, these will be running 24/7).
As far as pump(s) size is concerned, you will want to flow around 5 times the total volume of the aquarium through your filter per hour in freshwater applications. A 125 gallon aquarium will need a setup that pumps somewhere around 625 gallons per hour (gph) at the return. Quality pumps will have a chart showing you how many gallons they move at certain levels. Simply find a pump or combination thereof that moves 625 gph at somewhere around 5-6 ft. Personally, I like to go a little over 5 times per hour when considering a new pump. Usually, I try to get a pump that moves around 7 times the aquarium volume per hour. Over the course of several years, your pumps will begin to wear, moving less water. Your larger pump will compensate for this and allow you a few more years out of your purchase.

 

[bartman]

Thanks for the info... So let me see if I got this right.. The head pressure is not the point the water is released into the tank, but the water line in the tank regardless of where the tank was drilled.

You were correct about the 2 types of pumps...but I still am confused about that. When I contacted REd Sea they said the pump goes before the filter so does that make it a pressure or flow pump I need?

The configuration with 2 pumps is one for each canister, but I am thinking of changing that unless I can find some external pumps that would push enough 650 -1000gph at 50watts. I also plan to divert some water off to a fluidized bed for biofiltration Its a lifegard 600. I also have 2 lifegard heater units that need to be put in line. Thats why I need the 1000 gph. I hear these add feet to the head hieght. I was looking at the RIO 14 or 17 Hyperflow. They are rated high pressure at low wattage.

 

[FishSeller]

Bart,
You are right in assuming that adding more filters to your pump will drop your flow. IME, you lose less pressure if you put your filtration after your pump, not before. Most pumps are designed to push water, not pull it. Fluidized filters usually require a pretty heavy pump, as it has to push through water and sand simultaneously. I would think that running two canister filters, a fluidized filter, and two heater modules would need two pumps. One pump of a reasonable size would be working pretty hard to move water through all of your equipment. All of the added strain puts more pressure on your impeller than usual, and would certainly shorten the life of your pump.
If you're running freshwater, you may want to consider buying an appropriately sized wet/dry filter. You will cut down on the number of necessary "parts" and have more than adequate filtration as well as a place to put your heaters, carbon, etc. However, the setup you have right now would be killer for a planted tank. What would be cool is if Pentair (Lifeguard) made a cylinder for a Co2 reactor. You could put everything under the tank!

I do know for certain that if you are pushing your water through a filter, you are most definately running a pressure system. A flow pump is designed to move water only, not filters.

As far as your return goes, I'm assuming your water is returned at the top of your aquarium, unless it's drilled to return water on the bottom, the middle (both of these practices are rather risky), or your water return wraps around the top of the tank and exits somewhere below the surface of the water. Either way, you want to measure the highest point your water has to travel before entering the aquarium to determine head pressure. On that note, pressure increases as your water turns corners as well. Take into consideration how many 90 degree elbows your water will be turning before it exits into the aquarium.