I do not possess dissolved oxygen meter, unfortunately. All I know/guess is based on literature and personal observations.I'd be interested to know what we can do to maximise dissolved oxygen
In general, DO is a very volatile variable. The most important consumers of oxygen are microbes, primarily heterotrophs. Therefore we may say that dead organic matter (both particulate and dissolved) is the scavenger of oxygen.
To increase oxygen content, we should restrict feeding fish. Even if you make a one-day break in fish feeding you can help the situation significantly. In long-term perspective, modest stocking by small fishes helps the oxygen balance.
Mineral loading (fertilizers) increase oxygen demand indirectly, too. We say that majority of our microbes are heterotrophs, but that means no more that they consume reduced, i.e. organic carbon. As for other elements, they definitely don't refuse them if served in mineral form. Now, it is quite a common situation that a lot of organic matter is a source of reduced carbon but poor in other nutrients. Such an organic matter is not very attractive for microbes - they can't live on sole carbon. But if you add other nutrients into the habitat with fertilizers, they can saturate their needs for other nutrients from these mineral compounds and take reduced carbon from dead organic matter which was - until now - unattractive. In such a way, we can boost microbial activity, proliferation, and oxygen consumption.
What are the sources of oxygen? I believe plants are generally more important than gas exchange at the water surface. Plants (and algae) can make water oversaturated with oxygen (more than 100 per cent of oxygen dissolved in standard conditions). Aeration can increase gas exchange but, as a rule, cannot oversaturate.
The problem with plants (and algae) is that during night they turn into net consumers of oxygen. In the dark period, therefore, gas exchange on the surface is the only source of oxygen.
A dead fish, mollusc(s), shrimps may increase oxygen consumption dramatically. Temperature rise can increase microbial activity while oxygen solubility decreases. Various situations lead to die-offs of plants, algae, microbes. This is followed by increased demand for oxygen.I'm still a bit puzzled as to what caused the oxygen crisis).
Do you think roots, peat moss, herbs etc. can change pH by anything else but chemicals?I would like to attempt lowering the ph without using chemicals, ie roots, peat moss,
The disadvantage of natural materials is that they are never pure, so apart from desired chemicals they release some undesired or unknown ones at the same time. Par example, organic chemicals which increase oxygen demand. That is not to say that natural herbs etc. are always undesired.
On the other hand, pure (laboratory) chemicals have no unintended effects. If you decrease pH by sulfuric acid, you know that you add sulfates and turn bicarbonates into free carbon dioxide, period.