Jalal,
I would guess that you are getting a whirlpool from the 2" line to the surface of the sc. This is letting the pump suck in air from the surface. If this is the case you may be able to stop by putting a 45 degree elbow on the intake pipe at a slight downward angle.
Mike

Hi Mike. Actually on the 2" line from the sc I have a fernco fitting with a 45 elbow and other piece of pipe to centre of sc so that the out draws low enough down in the chamber. On this pipe I have a basket strainer fitting with a 16" piece of plexiglass glued over the fitting--looks like Aquaart's anit-vortex device. I did this as last year I did have a whirlpool going into the fitting before it went out into the pump. No air bubbles under the plexiglass as can see the basket fitting. The basket is threaded onto the pipe but I did not use any teflon tape.

If I switch this pump out into the higher head waterfall (has about 4 ft of head) I don't get as many bubbles in the priming pot. Tried this pump on the sc line to pump to smaller waterfall which is a 50 gal drum buried in the ground full of springflo. From the ball valve on the outside of the pump there is 18ft of 2" pipe to a 45 elbow another 7 ft of pipe to 90 elbow and intake 12" up from the bottom of the barrel. Ground level to top of barrel is the barrel height--maybe 3ft??. I call it the smaller waterfall as it's not as high. I thought I should have the main mechanical (sc with matala mats) running to the this biofalls as the higher one is one of those little giant filter falls and doesn't have as much media in it.

I am waiting for the priming pot for my old Sequence 750 pump (the one with the bent shaft) and was planning on running it on the skimmer line. When I sent this pump to Canadiankoi...and they tested it they said it was pumping at about 3600 gph instead of the 4200gph should be pumping. This rate would probably be better for the main biomedia right?

I am wondering if the oring is not greased properly. Lost my tube of oring grease and the local hardware store sold me plumber's grease. Maybe it's not sealing the lid properly???

Thank you so much for your help! Was 30C on Tuesday here-about 25 yesterday and raining today! Risk of frost tomorrow night and temps around 8C on Saturday. Had no algae until I started up the pump and noticed today pond is slightly green! Hot temps on Tuesday probably did that! I have very little plant cover yet as water lilies are just starting to take off and the few parrot feather and hyacinth I could afford need about 50 more! Not in my budget this year!

Okay you just gave us the probable answer to the problem. I would guess that if you shut the pump down after a few minutes you will probably see some water dripping from where the primer pot is screwed on the pipe. If you remove the primer pot and tape the threads you will probably stop the problem. The plumber grease should be just fine.
Mike

Sorry Mike. The fitting into the priming pot on both sides have teflon tape -the good stuff (my son is a steamfitter so gave me some of his industrial teflon--can't buy it locally). The fitting that is not wrapped is in the settling chamber. Do the rubber gaskets in the priming pot fittings need grease? I know the oring in the lid does but havn't put grease on the union thing on the priming pot. I checked to see if there were still air bubbles in the priming pot and there were so I shut down the pump as have to add some more foam to that waterfall when it stops raining! Changed out the one waterfall as well and had put foam in but somewhere the water is going under the big rocks I have along the edge and up in between the liner at the back of the rock. I have enough liner there so the water is going down into the pond but I'd rather it go down the waterfall channel than under and behind the rock! Isn't ponding fun!

I have one other thought that may be getting air in the line. Even though my pump box is 21" deep by the time you put in that priming pot the water level in the settling chamber may actually be lower than the piping into the priming pot. The line comes straight out from sc but I had to put a 22degree elbow then a short piece of pipe and a 45 before the priming pot. If that area is higher than water level from sc would that cause some air problems? I think I remember you talking about putting an air bleed valve in the line and maybe that's what I need to do even if the line isn't higher.

Thanks again Mike. Gotta go clean house as too wet to be outside!

Jalal,
What is the brand of primer pot? If you don't know the brand please describe the lid on it. A couple of other things to check is the primer pot has a drain plug on it. Remove it and check and see if it has a rubber gasket and that it is tight. Also check and see if the Oring on the lid doesn't have some debri caught on it. I think you are saying that there is a union built into the primer pot and you wonder if the gasket in this union needs grease. Normally it does not but even if this gasket was leaking you would not see air bubbles in the basket but the pump would be getting air. Being above the water level would not make any difference.
Mike

90 inch capacity strainer basket from Aquaart. Clear lock ring type lid, 2" ports slip female combo unions entrance port, male threads for union collar, 2" NPT female inlet. It's black and yes it does have a drain plug but I don't remember the drain plug having a gasket on it. Will check tomorrow.
Is the pump drawing water faster than the pot can fill up -too big for the sc? (300gals)

If the pump is too large the sc would just go down farther until the pump was sucking in air.
Mike