“My pool was commandeered by frogs this spring. I have the remnants of the frog fest in a tank. They are no longer tadpoles but pollywogs with the beginnings of legs. They are happy feeding on lettuce at the moment. Can I feed them turtle food when they mature as I read somewhere else in lieu of live bugs?”
Different kinds of tadpoles have different diets, but often the most common kinds of we find in our yards (or in your case, your swimming pool) are generalists and feed on a variety of foods.
Lettuce or greens will be consumed but are not the healthiest option, and in some cases the resulting frogs are not as large or strong as those fed other diets. Commercial freshwater aquarium fish flake is a better option, or alternatively or in addition to you can use foods designed specifically for tadpoles that are sold at pet stores. Algae wafers, shrimp pellets, and frozen fish foods can also be used, or like you suggest aquatic turtle pellets.
Many tadpoles will also feed on algae, detritus and films that naturally grow in aquaria.
Because the diet of many tadpoles is poorly known, providing a variety of foods may help ensure nutritional requirements are met in captivity.
Once legs grow tadpoles continue to feed and may even appear to increase the amount they are eating. It is when front limbs emerge, as you noticed, that the diet changes.
When front limbs are noticed, tadpoles need access to land and an easy way out of water. They will soon absorb their tail and while doing so will not feed. Instead they will live off the nutrients from the tail as their mouth parts change to switch to a carnivorous diet.
Juvenile frogs will need lots of tiny live food to survive. Small crickets and flightless fruit flies are the most commonly available feeder insects that are small enough for young frogs and toads to eat.
For the tiniest of frogs you can also look into springtails (Collembolans). Another easy option is using food collected outside. If you have access to pesticide/herbicide free forest and can collect handfuls of moist leaf litter, the leaves and scrapings from the forest floor often contain lots of little arthropods that especially tiny amphibians will eat.
Unfortunately, prepared or pellet diets will not be eaten by almost all frogs. You will have to switch to feeding live insects. You may be able to try other options later on, but frogs eat insects and usually need them to be live.