However you want to group them. Depending on fish you can certainly have more than one fish in a container based on size and compatibility. There's a bit of subjectivity there so hard to say. Maybe others can chime in on their experience there. Any preset heater will do as you're not asking a lot from it as it's small volume and not continuous duty. Once you're done in 14 days it'll sit on a shelf until your next batch. Below is a link to a lame video I made of my setup. It's just one example, there are far simpler and cheaper ways to go. My friend lives in a small apartment and he literally sets up 5 gallon buckets in his kitchen lol but I can't argue his results! This just gives you an idea. I'll be running my first batch through this setup beginning of February to time the end of my fallow period I'm my DT.