I spent months slowly establishing a new aquarium and in the last two weeks almost everything has died

I had a little 15 gallon aquarium with a small population of tetras, kuhli loaches, a betta, and two shrimp, plus snails. I bought a 55 gallon tank to give them all a better environment.

I filled the tank and planted it heavily then gave it a week to stabilize before I added anything. I moved snails, the shrimp, and some microfauna over. I bought a bag o bugs to increase microfuna diversity, and a dozen more shrimp, mostly young, and gave that another months to stabilize. The shrimp were growing up, carrying eggs, looking good to go. I added my five tetras from the old tank and everything seemed great for a few more weeks. Everyone was healthy, plants were thriving, baby shrimp were starting to appear. The loaches went in a little later and they've been good all along.

I went to a nice local aquarium store and bought five more tetras. Four days later, all but two tetras - one new, one old - were dead. I have no idea why. Water tested normal, so the ones from the store must have introduced something, but it was outside their return policy. But the shrimp were still doing well, so I moved the betta over. That was this week.

Today I go to check on my baby shrimp, and not only can I not find them - I can't find any shrimp at all. Usually they're all over the place. I could identify most of them individually because I bought a bag of wild colored, so they were very varied. Every single one of them died sometime between this morning and Wednesday night.

I guess it's a temperature issue. It got chilly here for August the last few nights, into the low sixties, and I didn't have a heater in the tank yet.

I'm feeling extremely discouraged. All I want out of these is to be healthy environments where animals can live safe little lives. And this was my biggest, most careful, most planned out aquarium ever. And now it's basically a graveyard. There's no shrimp population to even rebuild from. I have utterly failed in my mission to create a good environment for my little sea creatures.