Your "small mouths but a big appetite" is clever. Their intestinal track is likewise of small diameter, but long - indicating that they can utilize vegetable material and need some roughage to avoid infections in an empty intestine. That "small mouths" is a clarion call to feeding several small, quickly eaten portions a day. Fed in three feedings, the same amount of food will be better utilized and digested than if it was to be fed in one feeding.

Notice that while water which in which freshwater fish are found in has a number of different electrolytes or mineral salts, those salts are overwhelmingly calcium, magnesium, and potassium. Very little sodium chloride is found in freshwater.

Indeed, quoting Wright Huntley, a very accomplished and experienced aquarist, "Pure sodium chloride in RO water is essentially toxic.  I believe this is because cell transport requires the interaction of two or more electrolytes to be
effective (or some such story). They need to be in some very rough balance, I think.

In particular, I think I once read that potassium and sodium ions are very interactive in cellular transfer processes. You also really need at
least a little Ca++ and Mg++ plus a host of trace elements to make water really healthy for plants and fish. I refuse to count on fish food to
give me those.

I nearly killed all my plants and made my fish very sick, when I moved to Modesto (CA) and treated the pure Sierra snow runoff (<50ppm) with salt to raise TDS a bit. They all bounced back rather quickly when I added "Equilibrium" from Seachem."

Some shops will salt a tank when signs of skin parasites manifest themselves and the increased aquarium salt in the water irritates the skin of the fish. This causes the skin and gills to increase the production of mucus and that mucus fences out or encloses some skin parasites. Note that it also makes breathing through gills more difficult.

More troubling is the practice of salting heavily
stocked aquariums. It is good in that those tanks often run at a high levels (20=PPM ?) of nitrates. Some salt enables the fish's blood to continue to carry oxygen and avoid nitrate poisoning. They need to increase their partial water changes though. Long run there is no substitute for partial water changes and if the water isn't regularly changed, there is a danger that we are getting fish, potentially weakened, which have been putting up with nitrogen poisoning.

Someone else recently reminded me that plain sodium chloride does nothing for plants except maybe hurt them. Additionally, unless it is marine salt, it does nothing to buffer the water and may leave it vulnerable to a pH plunge.

In some cases (here's where one of those hardness kits or pH meters would come in handy) floating the bag in a tank (or floating the fish in a jar) and then emptying all of the shop water into a bucket (maybe pouring it through your fingers) and then gently putting the fish into the tank is better than fiddling around with the fish for a long time. That is especially crucial if the water has that funky, highly organic smell. (And I stand corrected on this one because I have been pretty casual about adjusting from from water like that.)

Dumping the LFS water keeps additional disease organisms out of the aquarium. Sure some stuff comes in on or within the fish, but in a world of equilibriums why bring home any more than we have to. :)

The exceptions here are when we are bringing fish from one water chemistry to one which is quite different (where there are great differences in hardness, TDS or pH) or when we are adjusting fish which are known to water shock easily. For instance, some marine aquarists used to put a very slow trickle flow on a bag of water, adjusting the fish to the aquarium water over a matter of hours.

I have begun putting some fish in covered jars, pouring out all but maybe 3-4"/ 2 cm of water. That much is added to the jar from the tank and I amble off to answer e-mail or peruse Guppylog or sop the hogs and feed the chickens. After 20-30 minutes, half the water is carefully poured out of the jar into an "out" bucket and that much is again added to the jar from the tank. That may even be done a third time.

It was done on a bucket scale, but despite efforts of that sort, I lost some black banded sunfish a while back. Even when we know what to do, we don't always do as well as we had hoped.

Sigh. There is always more to learn. And then we need to apply it correctly.