I highly recommend the YosemiTech DO sensors, which use optical sensing technology for long-term stability. https://www.yosemitech.com/en/product-cat-11.html
They use Modbus communication and can be user-calibrated. Here are their two main models (the Y505 is new and not on their website).
Y504 (standard model):
~$650 each (plus shipping plus 25% new China import tax)
±1% DO accuracy
±0.2°C temp accuracy
5 m standard cable length (10 m at additional cost)
10 s response time
Y505 (low-cost, aquaculture model):
~$450 each (plus shipping plus 25% new China import tax)
±3% DO accuracy
±0.5°C temp accuracy
10 m standard cable length
40 s response time
no protection cap over sensor surface
There are a few minor drawbacks to YosemiTech. They require purchasing with a bank transfer rather than a credit card, and their documentation isn’t great. However, we’re trying to help them with the later by reposting manuals and other info at https://github.com/EnviroDIY/YosemitechModbus. Overall, however, I find their sensors to be of excellent quality, and their email response time to questions (and also a warranty repair) have been quite good.