Hello all! I have been a fresh water fish hobbyist for the past three years and I have referred to the website many times, thanks for being there! I have two 29 gallon tanks, both planted with java fern, java moss, pennywort and anubis. One tank is home to 8 neons, 3 honey gourami, 3 harlequin rasboras and a bristlenose pleco. The other tank had a single female plattie, a couple of male guppies, six candy cane tetras and a bristlenose pleco. Today I started to remove water from this tank to do a water change and within a span of ten to fifeteen minutes the guppies and the candy cane tetras were all dead! I had not replaced any of the water yet. The pleco and plattie are fine. The other tank that I cleaned after the shock wore off somewhat is fine. Can someone give me some idea what the heck happened?