Charlie Spitzer wrote:
soap

why are you concerned about surface tension in the first place?


Charlie's right. In a freshwater aquarium, the only way you will
deviate from "normal" surface tension is by adding a surfactant like
soap or dawn or laundry detergent etc. But then your fish will be
injured or dying...

Surface tension does not play a role in oxygenation.

Oxygen gets into your tank primarily by diffusing into the water
at the surface. If the water is still, the layer at the surface gets
more saturated with oxygen, but the lower depths can become oxygen
defficient. That is why water movement is important. It is the
water movement caused by bubbles (from an airstone) that promotes
oxygenation and not the bubbles. They are small and have very little
oxygen in them compared to the rest of the atmosphere that interacts
with the surface of your water comlumn.

In overstocked tanks when the power goes out and there is no pump
or filter or airstone to circulate the water, fish can suffocate
to death because they will use up the dissolved oxygen faster than
it can diffuse into the tank.

On the other hand, if you run a sump/trickle tower arrangement,
you will never have to worry about low oxygenation because
the large surface area of the biomedia promotes very efficient
gas exchange.

See: http://cichlidae.com/articles/a014.html

JLD