The mullet that I have dealt with are big on grazing on algae so you may want to allow the pond to grow a healthy coat of it first. I would also feed them 2-3 times/day with spirulina flake or sm. pellets and another type of meaty small food. They have fairly high metabolisms so it can be a challenge to keep them at a good weight, but it can be done. They are also jumpers so watch out.
As far as adding salt, I would shoot for 10-20ppm for these guys (not sure what that is specific gravity). Watch out for evaporation to - that will send your salt up pretty quickly.
Here's the formula we use for adding salt (instant ocean) to our aquariums:
3.8 X gallons X desired ppm increase = g of salt
also 1 kilo = 2.2 lbs
For example: if you had a 200 gallon aquarium that was at 15 ppm and you wanted to raise it to 20 ppm this is what you math would look like:
3.8 X 200 gallon X 5 ppm = 3800 g = 3.8 kg X 2.2 lbs/kg= 8.36 lbs
Hope this helps,
Sharyl
