How would this hold up for saltwater aquariums. Notably, used in the sump within the cabinet which can be a humid environment ?

The problem with blue lab is the led dies. I have used 4 of these and after a year or two is a bar will go out and you cannot read the display any more. Do not buy!

can the probes stay completely submerged inside the reservoir at all times?

I've had mines for about 4 months now and it's always been in my reservoir 24-7. No problem so far.

Are the probes waterproof? Can they be used underwater?

Yes the probes CAN be submerged and are designed to stay in your liquid for extended periods (should be cleaned once a month)Youtube video from Bluelabs.com shows the EC/TDS/ppm probe submerged and this link from the manufacturer clearly states that you can completely submerged the PH probe:http://www.getbluelab.com/site/bluelabassist/files//Care%20and%20Use%20of%20Bluelab%20Soil%20pH%20Probes%20Aug09.pdf

ASIN	B0CCQDBZ3L
Battery Description	No batteries required
Best Sellers Rank	#62,847 in Industrial & Scientific ( See Top 100 in Industrial & Scientific ) #58 in Lab pH Meters
Brand	Bluelab
Customer Reviews	3.0 3.0 out of 5 stars (2)
Is Discontinued By Manufacturer	No
Item Weight	1.85 pounds
Item model number	BGUNLN02
Manufacturer	Bluelab
Product Dimensions	5.2 x 3.62 x 8.58 inches
Smart Home Compatibility	Smart Home Compatible
Temperature Accuracy	0.1 degrees_celsius
Temperature Reading Accuracy	plus_minus_0_point_1_degrees_celsius
Upper Temperature Rating	95 Degrees Celsius

Bluelab Guardian Monitor Inline Wi-Fi for Real-time pH, Temperature, and Conductivity (TDS/PPM) Measurements in Water with Calibration, 3-1 Digital Nutrient Meter for Inline Dosing Systems

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