I have an outdoor pond of 2000 gallon capacity, with plants and fish.
Proper maintenance includes periodic additions of a liquid culture of
non-pathogenic bacteria, which I purchase. Is there a convenient
method whereby I could grow my own supply by adding some of the
commercial product to a sterilized medium of my own making, such as
beef broth? I am not a bacteriologist, but am a chemist, and would not
be discouraged by technical info.
Thanks, monroe22

Theoretically, there is always a way to grow your own bacterial
cultures.  It would be helpful to know what the bacterial species is
so that specific culture requirements can be found.

telnady-ga:  The commercial product is proprietary, gives no info
other than non-pathogenic. Having the microorganisms identified by a
lab would be too expensive. Will have to use educated guesses.
monroe22

Your comment about the prohibitive cost of testing has urged me to
revise my comments here pertaining to your case.  There are 2 issues
to consider here:

1. The specificity of the product.

Bacteria and funghi are everywhere in our environment.  While you can
grow any bacterial culture yourself, it takes at least a modestly
equipped lab to ensure that what you are growing is indeed ONLY the
bacteria desired, and not other contaminants that may be circulating
in the room.  Although these bacterial contaminants may be relatively
harmless to you, it's anyone's guess what they would do to the
micro-environment of the pond in concentrated form.  Most labs use
antibiotics, selective nutrients, and minerals in their media brew to
selectively grow one group of bacteria while inhibiting unwanted ones.
 When these media are not available (or when they don't know what they
want to select against as in this case) they must use ultra-sterile
techniques to ensure that what grows on the less selective media is
indeed only what they set out to grow.

2.  Cost.

While basic media is relatively cheap, the more selective media can
get quite expensive (especially ones with antibiotics).  Furthermore,
cheap versions of the media need to be prepared by the end users (from
powdered form) and generally needs a fairly sterile environment.  It
is possible to buy ready-to-use media, but again it becomes quite
expensive.  It comes down to a dilemma between spending on lab
equipment (autoclave, beakers, Petri dishes, loops, etc) or buying
glorified media that comes ready to use.

You will also need to buy or improvise an incubator capable of
relatively sustained temperatures (whatever the temperature of the
pond is would be a good indicator).

It makes you wonder whether the proprietary mix is a justified cost after all!

Here are some links for purchasing media:

www.difco.com  
www.hardydiagnostics.com
www.ravenlabs.com

Good luck.

telnady-ga:  Many thanks for your astute comments. I believe I will
continue to purchase the commercial product and forgo a new career as
a microbiologist.
  Are you a GA Researcher? If so, post your comment as an answer and I
will pay for it.
monroe22

Dear monroe22,

Thanks for your comments.  Unfortunately, I am not a GA researcher yet
(I plan to apply when they start hiring again).  Nevertheless, it was
a pleasure to be able to assist you as it also helps keep me sharp.

telnady

telnady-ga:  You have been most diligent and helpful. I wish I could
compensate you for your advice. I hope you are someday a GAR.
monroe22