Ok. I have worked in the Balance feild for many years and have worked on a lot of water pump systems. It sounds like your Impeller is cavitating at start up. " Moving faster than the water can move " Which WILL cause the Micro bubbles that is caused by water being turned into a vapor which is Oxygen. Once the water flow is up to speed with the impeller, the bubbles stop.
You can try to lower the pump into the deepest part of the tank which will put more Inlet pressure due to the debth and may help it but I doubt it. You really arent going to get much of a pressure change in less than a foot of distance. Try removing the inlet pipes and then start it to see what happens. It may solve the probelm or make it a little better. If you restrict the output, it may just make it worse. As stated above, I have some MJ1200's that due the same thing from time to time when the filter gets dirty and restricts the input flow.