The stuff in the bottom of the pond is likely a really good source of nutrients for a garden or lawn. If you stir up the whole thing with the water in it, then you'd probably have a serious plant booster solution. Like super
compost tea.
I'd let the frogs be as they are probably eating a lot of bugs and larval pests. Their young (tadpoles) will help clean the pond of algae and other gunk. Even if you kill them off, they will be back every year. It's good habitat for them and as long as you're going to keep fish, the frogs are a bonus feature. Just my two cents. Frogs are a sign of good clean water, because they have a wet skin they are really susceptible to foul water conditions. Think of them as a good omen. It's essentially already ready for fish. Might just need a filter to better oxygenate the water.
I'd probably line the edges with filter type plants that help clean the water. You could look into Natural Swimming Pools to see how they go about filtration. It's pretty cool.
best of luck
Also. The wall looks like it's too high for a frog to get out of the pond once it's inside. Perhaps placing a log in there so that they can climb out would be of some use as well.
