Alright, I need some advice. Ive had tropical fish for almost 15 years now but and have never had this problem. Two days ago I was doing a biweekly water change on three tanks that I have in my apartment. Two of them are ten gallons and one is twenty. I use the python to make this process more effecient. I changed about 20-25% of the water while cleaning the gravel. Then filled the tanks back up. The next day(yesterday), I lost eight fish in the community twenty gallon, and twelve in my roomates community ten gallon. My first guess was that something in the water killed them. Possibly I high conentration of chlorine yet I never had the problem with the city water till now. Yet to my amazement, in my ten gallon tank I have guppies with about 30 month old babies, and I didnt lose a single one of them. Now I cant understand why all of the other, older more adapted fish, would die and not the fragile baby guppies. You figure they would kick the bucket first. Anyhow, the fish that I lost in the twenty where all of tiger barbs and red eyed minnow things and no other species. And in the ten gallon I lost all of the neons, save two, and no other species. I cant figure it out. Any imput would be greatly appreciated before I decide to do another partial.