The aquaponics system that I built consists of a sump tank with a pump that pumps water up to the fish tank (for pictures see Project #2 – Aquaponics). For water quality purposes the recommended exchange of water is 1/2 the volume of the fish tank every hour. So for my 50 gallon fish tank that comes to 25 gallons per hour (gph). For higher fish densities, the recommended turnover rate is higher. I’ve described how to determine your recommended fish tank volume and growbed size, which has a calculator that also shows your recommended pumping rate.
I happened to have an old 80 gph fountain pump, but realized that this just didn’t have enough power to pump the 25 gph I needed up to the fish tank. In fact I wasn’t able to get any flowrate from the sump to the fish tank using this small pump. The reason I wasn’t able to get the flow I needed is because the pumping head was too high for the pump. When the pump didn’t have to lift (or push) the water up to a higher elevation, it was able to get the 80 gph it was rated for. By connecting the discharge of the pump to a tube and raising the tube 4 feet in the air, however, the flow slowed to a trickle. Raising the end of the tube any higher cased the flow to stop. The ability of pumps to push more water at lower elevations is typical, and can be seen on a pump curve.
In order to pick out the correct pump that you will need, I suggest the following steps:
- Determine the amount of flow you need for water quality purposes. (see recommended fish tank volume and growbed size). If your system is set on a timer, instead of a constant flow, then you will need a larger pump to take this into account.
- Determine the amount of “head” you will need to pump against. This will most likely be mainly the elevation difference the water must be pumped up to. Other things that add to “head” are major and minor friction losses (see my description of headloss and pump curves).
- Compare the pump curve to the amount of head you calculated in step 2. The pump curve will show what the pumping rate will be at that head.
Many small fountain pumps don’t have readily available pumping curves. If that information isn’t available, look for a total pumping height that it can pump to. You can expect little to no flow at this height.
Below is a picture of the pump that I bought for my aquaponics setup.
It says it is rated for 315 gph, but notice that it says 5.9 foot maximum pumping rate. I knew I would be pumping about 5 feet. Once I put the pump in place I measured the flow and it pumped about 78 gph. So, I measured the flow to be more than double the minimum recommended for my tank size (50 gal). If I think the water is cycling to much, then I’ll turn the pump down, otherwise I plan to leave it at this pumping rate.
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