These are a couple of scenarios that could happen to your pond that may encourage you to check the dissolved oxygen level rather quickly.
- You notice most of the fish at the surface during early morning hours trying to get oxygen
- After a heavy rain storm or after the ice melts you see dead or dying fish
- It’s a hot summer and the pond is loosing water due to lack of rain
If you are fortunate to own a dissolved oxygen meter with a probe 
or can hire someone who can help you, start with lowering the probe to the bottom and slowly bring it up to mid-depth. If you measure less than 4.5ppm you need to act quickly to supplement the oxygen level in the water by creating surface turbulence with a pump or start up the aeration system that you are happy to have. It still might be too late, but some of fish and aquatic critters will appreciate the help.
