The smaller is the tank - the higher the nozzle is supposed to be
attached. With my 5.5 I was supposed to go with the topmost (shallowest)
attachment point, but I ignored this recommendation and went with the
deepest one to give bubbles more travel time. Does anybody know what is the
importance of this recommendation in the installation instructions?

The deeper you put the diffuser, the more time the CO2 has to dissolve in the
water. More isn't always better with CO2. You can poison the fish. It's not
likely with DIY CO2 in a large tank, but with a 5.5 gallon tank, you have to be
careful. Measure your pH and KH, check the charts, and make sure you aren't
poisoning your fish.


Leigh

http://www.fortunecity.com/lavender/halloween/881/