Two tanks, run by one canister filter

["Greger Lindstrand" <greglind at algonet_se>]

Inspite of advice against it I say it can be done.
All you need is "comunicating vessels". To achieve this you use common plastic plumming pipes of the type you use underneath the bathtub or kitchen sink.
You need 3 pipes and two bends. Put them together in a "U" shape and lower the "U" into your tank so that it gets filled with water. Put 2 drinking glasses (one at each) over the ends (under the surface) so no air is present.
Turn it over ("n" shape) and lift it up over the water, holding the ends in the glasses. The water inside the tube will not run out (laws of fysics).
Lower it again so that you have one end in each tank and remove the glasses under the water surface.

Now you have 2 communicating vessels that will always strive to keep the surface at the same level.
Put the inlet from you Ehiem in one tank and the outlet in the other and start. Water will flow into one tank - thru the "n" bend into the next and then thru the outlet into the Ehiem again.

Due to the larger area of the tubes (40-50 mm). A greater amount of water can flow in a slower pace, than your Ehiem can pump into the first tank. The larger plumming you use, the more safety against overflowing you have and the slower the pace of water flowing between the tanks, will be.

If you don't put a net over the ends, some animals will find their way into the other tank (and back). Snails, algeaeaters, eels and shrimps will make the journey thru the pipe. Even smaller fish will take the trip without beeing harmed.

The only drawback with such a system is illness. If one tank gets a disease then the other one will to.

/Greger - Certainly not in T-shirt anymore. Summer is gone but there's always downhill skiing to look forward to :-)