Welcome to the forum.

Substrate is very important for many reasons.
It gives a surface for good bacteria to grow to help with water quality.But choosing the wrong substrate can also be detrimental.
These frogs are not very accurate eaters and can swallow some of the substrate.
If the substrate cannot be passed by the frogs digestive system it can be deadly for the frog.
Personally I prefer Sand as a substrate for others prefer bare bottom or other words no substrate at all.

Now for food.
I would not feed any freeze dried food as this can cause bloating which will make the frog not be able to submerge
And stress it out or worst case scenario be fatal.
I would offer frozen not freeze-dried. Blood worms, brine shrimp are good start.
With that being said having a large pebble or rock substrate food can fall in between the rocks and rot and cause serious water quality issues.
I would keep with the staple of the frog and tadpole pellets with a weekly treat of a cube of the frozen food.
You can also feed nightcrawler pieces cut to the size of the space in between the frogs eyes to start. Once a week.
As the frog grows you can feed larger pieces.

I have never used salt to treat illnesses in aquatic frogs. But I would discontinue the use if the frog is healthy.
What size is the tank.
If the tank is not large enough for a power filter I highly recommend A air powered sponge filter this will greatly improve the water quality.