There is a good ready-reckoner at The Goldfish Paradise site under the
utilities section. http://www.goldfishparadise.com/utl/calculator.php

When folks tell you fish need 10 gallons each remember that they
usually mean US gallons, which are different from UK gallons. Also
remember that a lot of these equations give you the maximum number of
fish. Its a limit, not a challenge. An slightly understocked tank
gives you a lot more lee-way for when things go wrong. (And with the
best will in the world things always go wrong at some time).


In terms of "oxygen weed" I presume you mean an Elodea type thing. A
little will soon grow into a lot. So long as the tank isn't too
congested it shouldn't be a problem. The plant will put oxygen into
the water during the day but will take some out at night. So long as
you have plenty of movement on the water surface this will not be a
problem though. The other thing plants can take out are nitrates - the
end product of the tank cycle. This is no substitute for regular
partial water changes though.