Yes, it sounds like fertilizer or salts have leeched into your water. I would buy a few gallons of distilled water and flood it through your carnivorous plants, it might not be too late to save them.

I would definitely not try and filter the water through other plants. The water would pick up not only fertilizer, but also minerals, salts, etc etc -- all things you do NOT want in your CPs.

Can you purchase distilled water from a grocery store? That is about the best to use, along with WFI, RO, and Deionized water.

I wonder where you got that idea that you could filter water that way & make it good for CP? X-D

Hope your plants are salvageable. If not, you still gained something - experience. Most growers learned by trial n error, what work best for them and what not. It seems it all depends where you live.

The way you were growing them is about right. You just went wrong about the water.

D. capensis is a tropical plant that doesn't require dormancy, they grow year round. Capensis are quite forgiving and easy to grow. For that reason they can become weeds very easily. Seems like your capensis went into shock when taken outside. It's a whole different environment indoors that outdoors so it took its time to recover.