Its pretty simple: if the shellfish are all dead and the tank is smelling like a Marseille sewer after the bouillabaisse festival? Then the water is too fresh.
Many shellfish will tolerate a decent range of salinity. They have to as in the wild salinities can vary wildly with the tide and discharge from local rivers.
For freshness/animal welfare (they are ...
You cannot (technically, yes, you could, but practically, no)
First, a store will not let you tamper with the tanks; either by taking sample or by prodding it with some tools (unless you are a real food inspector)
Second, who are you to know if the water is "good enough" ? Are you a food inspector? what guide will you use.
Third, Stores are not set up to ...
There are several ways to measure water salinity (water salt content), most are impractical for that situation. A very effective and quick way to measure salinity to use a salinity meter, these are small electronic devices which measure the conductivity of the water. You just dip it in the water and take a reading. More salt = more conductivity.
These can ...
For shellfish like clams and mussels, it's actually possible for a fishmonger to keep them alive. Keeping them in clean, cold, circulated SALT water is best, because that allows them to stay alive but dormant. Obviously, it doesn't get fresher than live.
For fish, this is not an option. Most fishmongers receive the fish already dead, so the best practice is ...
Temperature is the most important factor in shellfish storage.
Also, keeping shellfish in fresh water (compared to sea water) is not a good thing.
From the British Columbia Center for Disease Control (pdf)
"Do not put live shellfish in a closed container or into fresh water (the shellfish will suffocate and die)".
Anecdotal, most of the shellfish I've ...
The safety of storing your lettuce submerged would depend on what type of bacteria are on it before storage.
Anything from botulism to E coli could be hanging out in your wet salad.
Both of these, and other, bacteria can survive refrigeration, submersion. E Coli can apparently survive highly acidic environments and fermentation.
That said, it's the ...
I don't see any reason for it to be unsafe.
However, I think you are wrong in assuming it will keep longer. I keep my lettuce in a closed container with water on the bottom, then a trivet, then the lettuce on the trivet, raised above the trivet, and the whole thing in the fridge. It keeps that way for weeks, not getting yellow, and not wilting too much.
The way mold grows is, a spore has to land somewhere moist, i.e. with at least a tiny amount of liquid water on it. So if you want to keep stuff like fresh berries in the fridge for many days, the best thing to do is not wash them right away, but instead leave them with a nice dry outer surface. That way, even though they're obviously full of wet juice ...
Methods, from best to worst (assuming the bread is in a sealed plastic bag):
According to the FAQs for Dave's Killer Bread, which does not use preservatives:
Q: How should I store my bread?
A: The best way to store your bread is on your counter or in a bread box at room temperature. Take care to keep your ...