If you leave chlorinated water sit in a bucket for 24 hrs it will dechlorinate. Also you can buy chlorine remover from the pet store that quickly makes tap water usable. Yes the colder the better for the minnys. I used to feed them oatmeal. Also keep In mind how many minnows you have per gallon of water. If you cram to many at once. They will polute the water as you found out. Charcoal is the key to removing the ammonia. The filter just gives the good bacteria a fast place to grow and do its job. But that still takes time. So water changes are a necessity. That's why I used a 55 gallon plastic drum. To get technical, fish produce waste ( ammonia ) through their gills everytime they breathe. Good bacteria breaks down that ammonia into nitrites which is then converted to nitrates. That is why well established aquariums do well. They naturally break the ammonia down and convert it. But there also isn't hundreds of fish. So there is a balance. With minnows we want as many as we can get and keep to cut trips down. But a large gallon container is needed to keep the water quality from going bad quickly. The pet store also sells ammonia absorbing charcoal in big containers. I used a pantyhose foot and used half the container in it and hung it over the edge. Got the biggest aerator pump I could find and the biggest round stone for aeration. Then used a submersible powerhead to circulate the water. Along with regular water changes I kept 300 -500 minnows alive in my plastic 55 g drum. With minimal loss of fish.