Sorry this is a bit of a long one, but please stick with me here. And so you know, I'm trying my best to be a good fish owner but I'm still quite new at it.
I got a 2 gallon tank for Christmas last year and ended up getting a Betta just a few months after. This little 2 gallon tank was unfiltered and unheated. (The temperature was always at least 74, which wasn't much different from his pet store cup he was used to.) I made water changes just about everyday to make up for the lack of filter. My little guy was happy and mostly healthy, but I really wanted him to have heat and filtration.
I ended up buying a Tom Aquarium Products Mini Filter. It seems to have decent reviews so I thought it would be okay. As soon as I put it in my tank, he got extremely sick. It was a few hours before this was obvious so I quickly removed the filter when I discovered he wasn't feeling well. For almost 3 days he was on the brink of death(I'm talking barely breathing and laying at the bottom of the tank on his side), and somehow on the fourth day miraculously was alright. He got better but his water quality still wasn't what I wanted it to be.
I just assumed that the filter was just too new or something and he almost died from "new tank syndrome". I found a really amazing deal with a 10 gallon tank and filter not long after and decided I just wanted him in a bigger home all together. Unfortunately that didn't go as planned because the fish I used to cycle the tank are still alive and I'm pretty sure he would kill them. (didn't know about fishless cycling then). My 10 gallon tank is very stable and cycled. It's been running for several months with no problems.
I thought my 10 gallon would be a good way to fix my filter problem. I set up an old 5 gallon I found in the shed, used water and gravel from my 10 gallon and hoped it would make it safe for my little guy. After a month or so of using water from my 10, I bought a few new fish that I was going to transport into the 10 anyways. I basically used them as a test to make sure it was safe and they died the next day. I got a new ones with my receipt and they were getting really sick right away when I put them in. Avoiding further fish death I moved them to my 10, they perked back up and are still fine to this day.
I decided to move my Betta to the 5 gallon anyways, it's just unfiltered. (He has a heater though!)
I have no idea if something is really wrong with this filter or if I'm just missing something here. No matter what I do with this thing it always turns out deadly. Any advice would be great. I'm completely at a loss.