[abrogard]

 Don't know if this comes within the ambit of these forums.  If not I assume the moderator will delete the thread and I proffer my apologies.

 The query is: How to detect, measure, the dissolved oxygen in water?

 The practical application is an outdoor pond, or acquarium.  A small pond.  A bathtub actually, with four little goldfish swimming around in it.

 These are tough fish.

 But they need dissolved oxygen the same as any other for a comfortable life.

 I can aerate the water by mixing or fountaining a stream of water.

 But I never know when's enough.

 They make instruments to do this measuring and they can cost more than $1000 and a cheap one is $300, say.

  I don't know how accurate those machines are and what other things they measure.

 It is possible, I'm thinking, hoping, that they are far more accurate than is needed for this application.

 That there is some simpler, cheaper way that will give a workable indication of the amount of dissolved oxygen at any one time to a reasonable sensitivity as far as the fish are concerned.

 Is that a reasonable question?

[billnotgatez]

Your Question is reasonable.
I moved this post to this part of the forum because you seemed to be a hobbyist.
We suggest in forum rules you make use of search engines and WIKI.
That way you can come to us with questions of what you do not understand from your on line research.

I used the term
dissolved oxygen test
for my search and got lots of hits.

I used the search function in the selection bar at the top of the page and got hits from past discussion in this forum.
I used GOOGLE and got hits as well.

I thought this WIKI was interesting
http://en.wikipedia.org/wiki/Winkler_test_for_dissolved_oxygen
I also found this GOOGLE hit interesting
http://www.ecy.wa.gov/programs/wq/plants/management/joysmanual/4oxygen.html


Side note
ambit is an interesting word.

[abrogard]

 Yes, well thanks for that but I've been there.

 Took another look - quick overlook - after your post but saw nothing new.

 Many hits but none answering the questions.

 Probably my fault for not asking the question right.

 The 'practical application' is a fundamental part of it.  I should have made that clear I guess.

 I'm looking for an answer such as:

 For a quick easy test to see if there's sufficient oxygen for your fish ( i.e. within range a - b ) stick a piece of fishoxygen litmus paper in the water.

 or - and more probable, I admit:

 There's no quick, cheap, easy test.  About the cheapest and simplest that gives results within the range the fish require is the ABC test.

 That's what I'm looking for.