Comparing milligauss to tesla (a deadly amount of radiation) isn't actually fair.I've never seen Gauss as a unit before. I had to look up what Gauss measured. Well, it measures magnetic field strength. Not EM radiation.
With that being said, anything with a magnet inside it will cause the needle to spike. A lot of electronics will make the needle move too. But that's why the filter and pumps cause the spike.
10000 Gauss is 1 Tesla. An MRI exposes you to 1-3 Tesla.
The magnetic field from the equipment connected to the tank (.00000001 Tesla) is nothing to worry about. It'll produce some extra heat in the area. But not dangerous radiation.
Cell phones which only produce 5-10 milliguass have been found to disrupt sperm count and motility.
In this article published by the World Health Organization it states that a "pooled analyses of epidemiological studies demonstrate a consistent pattern of a two-fold increase in childhood leukaemia associated with average exposure to residential power-frequency magnetic field above 0.3 to 0.4 µT. "
http://www.who.int/peh-emf/publications/facts/fs322/en/
100 milligauss (what you get standing next to the aquarium pump) = 10 ut which is over 20x's the amount of radiation in the above study where children developed leukemia.