Originally Posted by Newkeeperotis
Is that the tank with the filter in the lid the filter system in those are supposed to be extremely good so I don't think it's filter related
If it is filter related issues we would be seeing spikes of either ammonia or nitrite. What are your nitrate readings? Those can be just as important as the ammonia or nitrite readings, especially since your water supply can contain large amounts of nitrates, especially when the farmers are fertilizing their fields.
How do the guppies act before they die? Are they floating at the top gasping? Are there any visible growths on them? Are they scratching against decorations?? Open wounds?