Chlorine is a huge problem, and just because you have a filter on the sink, doesnt mean it removed the chlorine. Doesnt even mean that the filter you bought necessarily does Jack shit really. A lot of filters can simply rip you off.

It does sound like chlorine poisoning. That, and I agree with Willow. Decor needs to be replaced after such a long period of time for safety reasons in ANY fish tank. Hopefully you get the problem fixed, but the first problem is the fact that theyre guppies, lol.

guppies are pretty irritating with the amount of health problems they come with. Id rather rid myself of that headache all in all and just restart if it keeps coming back. There are many other very durable types of fish that are more suitable for fluctuating water conditions, but guppies just fall way too easy for me to even consider allowing someone to pay me to keep them.

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Good luck! <3