The loach and molly should both reach about 4 inches long when mature and that along with the guppies and frog are probably all you want in the tank. You could probably squeeze in another frog if it's a small species. If you get rid of the molly you could add other stuff.
Post a picture of the loach and the algae. Most loaches don't eat algae.
Algae grows when there is an imbalance in the light, nutrients and plants.
Too much light or nutrients and the algae grows.
Not enough live plants to use the light and nutrients and algae grows.
Normally reducing the nutrients with big water changes and gravel cleaning, and reducing aquarium plant fertiliser (if you use any) can reduce the algae. Adding live plants will also reduce algae. But post a picture of the algae first so we know what we're dealing with.
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If you were going to add salt you should have done it straight away. Right now it's unlikely to make any difference unless more fish are unwell.
The best thing to do if fish get sick or after a fish dies is
1) wipe the inside of the glass to remove the biofilm (slime).
2) do a huge (75%) water change and gravel clean the substrate to dilute any pathogens in the water.
3) clean the filter if it hasn't been done in the last 2 weeks.