How to make sure guppies don't fall sick that easily?

[JasonRipped]

I have a few tanks of guppies and the big tank with the blue grasses are doing very well with a heater in it, the other tank with all the albino yellow tux seems to fall sick quite often. Is it due to the fact that it does not have a heater?

[saintdevilz]

I breed them outside my house without heater...