Mystery fish deaths.

I just bought a 10 gallon tank to put smaller SMB and Channel Catfish in before moving them to my 175g and 30g so they dont get eaten. Right now i have a good sized Grass pickrel about 12 inches in my 175 so i decided to use my new 10 gallon to put my 2 inch Channel cat in. I filled it and put in water dechlorinator it and waited 1 week. I then put in the channel cat and a mystery fish to keep him company. After about 2 weeks my mystery fish mysteriously died. Just thinking it was an older fish I did a 10% water change and left the catfish in there. The next week the channel cat got ich then lost most of it then next 3 days. after that he returned back to normal and stayed active mostly at night. 2 days later almost all the ich was gone with only 3 or 4 spots. I then did a 30% water change and used the water conditioner on it. the next day he mostly layed on the side of the tank and started to move around at about 6:00 PM like normal. after about 2 days of that i came home from work and he was dead. I will admit that i never checked the water for ammonia or anything. but i dont think a fish can produce that much waste in a 10 gallon after 1 month with me doing water changes. Can someone tell me what happened or what mistake I made. Im going darter hunting in a few weeks and do not want them to suffer the same fate.

Did you do anything to cycle the filter? A 10% water change isn't much to control ammonia, and if it's able to just build up over time it will reach a lethal concentration. A 2 inch fish in a 10 gallon tank is certainly enough to produce a deadly level, especially when the other fish died and produced an additional burst of ammonia.

I think if you put the 10 gallon filter on one of your big tanks for a couple weeks (don't leave it idle) the problem should not be repeated.

The filter was cycled. I should have wrote that before

The fact that a fish got Ich is telling - unstressed fishes do not get Ich. Even Irate's Tank O' Death ™ has not seen a case of Ich in 20 years or more...

Yeah, my first suspicion was an uncycled tank too. Cycled filter or not, I would look at that first.