Pond Boss Forums MANAGING AN EXISTING POND Evaluating and adjusting fish populations Where have all of the small trout gone ?

I have only used YSI brand. First one IIRC a 550 that used a membrane. To use it first you have to make sure that the membrane stayed moist when not in use, and then you had to move the probe at a certain speed (inches per second) to get it to read correctly. You couldn't just drop it in the water and leave it there. Also, when I first turned it on, it took about 15 minutes to stabilize before I could get accurate, repeatable readings.

I purchased an Optical YSI O2 meter. YSI ODO is what it was called. It's now out of production but they have comparable units. Again, you have to keep the probe in a moist environment (a rubber cap comes with it that you leave a bit of moisture in when stored) but the biggest advantage to me was it gets repeatable readings a minute or two after turning it on, and I don't have to re-calibrate it every 6 months or so. I check it, and it's never been out of calibration. I also don't have to keep it moving to get good readings like I did with the YSI 550. The ODO also has an easily removable probe, the 550 had a probe that was hard wired to the unit. I can use a short or long probe depending on the water depth, but I purchased a 10 meter probe when I purchased the ODO and it's been on the unit since purchase. It has come in handy to read deeper lakes and ponds. I was worried that I'd run out of probe length if I only had the 3 or 4 meter one.

They aren't cheap though. Maybe others (Snipe, Lusk, Cody, etc.) can chime in with what they are using.

As for readings, for warmwater fish, no less than 5 mg/l and for trout no less than 7 mg/l. It is very dependent on water temp. Saturation levels will vary according to temp. Many species of fish can tolerate lower O2 levels, but those figures I gave you are what I consider minimum levels. Adding O2 to the water without paddlewheel agitators, surface agitators or diffusing pure O2 into hauling tanks is a struggle once O2 levels drop below 4 mg/l. It's a LOT easier to keep it above 5 mg/l than to bring it up over that once it crashes below it.