Maybe add a another valve at the top to purge any air.
2 problems with the outlet at top that I see...
#1 Water movement slows since it has to fight the back pressure of pushing thru a larger column of water.
#2 Then the CO2 would go right up and out with very little chance to be assimilated into the water.
you could take another easy way out, and buy an inline heater housing, and install your own heater into it..
http://www.drsfostersmith.com/produ...ubref=AA&ci_src=14110944&ci_sku=0905882000000
I'm at work, so I can't make this fancy, but this is what I meant, it will push air bubbles out, if you are worried about that. (Turn the heater back right side up of course)
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