Depending upon how much room you have (or a place to hide it, under or behind plants, etc) you could make a 55G trash can filter. A DIY wood covering or something would also work, adding an extra 50G to the system.
It more or less is a giant filter. With lots of media to maximize BB area and multiply the water volume. Water is pumped into "can" (I used a plastic one) and then flows out when it gets to a certain height, pouring back into the pond.
Can she get a little bit bigger Pond / exchange it?
Guppies and Endlers are great! You might need to consider a hungry pond companion (to keep the baby numbers in check) or nice LFS
to take them to later when the population goes up too much.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves