For the past several years, I've set out up to eight 34-gallon pond inserts and several 74 liter "muck buckets". I put out all my marginal plants on the 3 shelves in each pond and fertilize them. You want to promote root growth, since these plants are your natural water filters. Once the water temps are warm enough, I also set hardy pond lilies into the water (and fertilize heavily to get flowers). I usually replace the water with fresh water before adding the fish, since heavy fertilization on the plants causes elevated ammonia levels in the water.
Marginals can grow as tall as several feet, and pond liliy flowers can be several inches above the water level. Pond lilies provide cover and shade, and can help keep "jumpers" in the water. You need both types of plants in a successful outdoor tub or pond. I think it would be difficult to your ponds with a framed-out structure. Pond netting may work, if you work around your tall marginals.
Each year, I find aquatic frogs in a few of my ponds, and spring is also the time for Eastern gray tree frogs. I check my ponds daily and when I find frogs, I net the aquatic frogs out of the ponds and rehome them in a local pond. The gray tree frogs occasionally do spawn in my tubs, and I net out as many eggs as I can. some, of course, remain hidden. The tadpoles eat algae and I've not found that they predate fry. The froglets leave the ponds as soon as they are grown, and they are very small.
Our Livebearers journal editor, Ted Coletti, has been summer tubbing for about 20 years, and has a Facebook page for summer tubbers. If you are interested, the name is "Northeast Fish Tub & Water Garden Study Group". Ted and other members share good information and beautiful photos of their ponds.