Adding a filter might reduce the colony in the other filter by spreading it around. I can't see how it would crash it unless it ran out of food (ammonia). Hang in there. It'll cycle back up in no time.
Yes the small tank has been stable for some time now. It was a 5g but I moved the fish and filter and filter and everything into a 2.5g about a month and a half ago and that cycle is still going strong. The only thing that could have upset the cycle on the 10g was that I added an additional filter, and I only did that to help the cycle! Its so annoying...wish me luck from here on out....sheesh
Just read your "make sense" thread again. Is that the 2.5g that''s been running smooth and stable for almost a year now? Did you check tapwater? Did you forget to treat your refill water? It only takes once. Any plants die? I can't think of anything that would crash your cycle without killing your stock.
It's some kind of mini-cycle. Treat it like a newly cycled tank. It'll come back in short order.