To be able to answer your question properly, we need more info: * how many guppies did you get, and when did you get them? * how many have died? (title mentions 'dying guppies' but content only mentions 'not swimming around') * how many are behaving oddly / ill? * any other symptoms? * are there any other tank inhabitants and, if so, what? * what size of tank is it? (in litres, plus dimensions) * what is the brand/size of filter? * what is your tank maintenance and filter-cleaning regime? * what dechlorinator are you using? * when was the tank set up? * how was it cycled? * what are your current levels for ammonia, nitrite and nitrates? * what is your water hardness in GH and KH, or, from your water supplier website, what is the CaCO3 and German degrees hardness?